Final? pass of finals

Add a least-upper-bound to set Phis to their respective Top while preserving final fields.
Requires a ReadOnly node to "cast-away-writable".
Series of isFinal/makeRO in all Types.
Changes tests to match r/w intent.

26 files changed, 426 insertions, 215 deletions

diff --git a/chapter17/README.md b/chapter17/README.md
index b7a1afd6..bf998ab8 100644
--- a/chapter17/README.md
+++ b/chapter17/README.md
@@ -15,7 +15,7 @@ Allow pre-increment for identifiers only, for now: `--pc`.
 ## var & val
 
 `var` can be used in the `type` position to indicate a "variable" (mutable
-value), whose type will be inferred from the initalizing expression.
+value), whose type will be inferred from the required initalizing expression.
 
 `val` is the same as `var`, except it is a "value" (not mutable).
 
@@ -30,78 +30,68 @@ infers as
 `S s? = new S;`
 
 
-## Initialized fields are immutable by default
+## Mutability
 
-Typed field declarations with an initializer are immutable by default, and can
-be made mutable with a leading `!`.  `var` and `val` keep their current sense.
-Fields are immutable at the end of either constructor, but can be changed
-during construction.
+Typed primitive fields are always mutable.  Typed reference fields without an
+initializer must be mutable to get a value.  References with an initializer are
+immutable by default and can be made mutable with a leading `!`.  `var` and
+`val` keep their current sense and can be used to make any field mutable or
+immutable.  Fields are always mutable during construction, but will become
+immutable at the end of either constructor.
 
+`int x; x=3; x++; // OK, primitive so mutable`
+`int x   =3; x++; // Ok, primitive so mutable, despite initializer`
 
-Examples:
+Assume in the next examples this struct exists:
+`struct S { int x; };`
 
+Then:
+`S s; s = new S     ; s.x++; // OK, no initializer so s.x is   mutable`
+`S s; s = new S{x=3}; s.x++; // OK, no initializer so s.x is   mutable`
+`S s    = new S     ; s.x++; // Error, initializer so s.x is immutable`
+`S s    = new S{x=3}; s.x++; // Error, initializer so s.x is immutable`
 
-```java
-struct Point { int x,y; };  // Fields x and y are not initialized, mutable
-Point p = new Point;
-p.x++;                      // Mutate is ok
-return p.x;
-```
+Leading `!` makes mutable:
+`S !s = new S; s.x++; // Ok, has '!' so mutable`
 
-```java
-struct Point { int x=3,y=4; };  // Fields x and y are initialized, immutable
-Point p = new Point;
-p.x++;                          // Error!
-return p.x;
-```
+'var' is variable:
+`var s = new S; s.x++; // Ok, has var so mutable`
 
-```java
-struct Point { int x=3,y=4; };  // Fields x and y are initialized, immutable
-Point p = new Point { x=x*x; y=y*y; }; // Ok to mutate in the constructor
-p.x++;                          // Error!
-return p.x;
-```
+'val' is a "value": not mutable through this reference, but may be mutable
+through other references.
+`val s = new S; s.x++; // Error, val so s.x is immutable`
 
-```java
-struct Point { int !x=3, !y=4; }; // Fields x and y are initialized, mutable
-Point p = new Point; 
-p.x++;                            // Mutate is OK
-return p.x;
-```
 
-```java
-struct Point { var x,y; }; // Error, var and val require an initializer
-Point p = new Point; 
-p.x++;
-return p.x;
-```
 
-```java
-struct Point { var x=3,y=4; };  // Fields x and y are mutable
-Point p = new Point; 
-p.x++;                          // Mutate is OK
-return p.x;
-```
+### References with an initializer are deeply immutable
 
-```java
-struct Point { val x=3,y=4; };  // Fields x and y are immutable
-Point p = new Point; 
-p.x++;                          // Error!
-return p.x;
-```
+References with an initializer are deeply immutable, but a mutable version of
+the same reference can exist.  This works for `var` and normal type
+declarations: `var s = new S;` and `S s = new S` both make `s` an 
+immutable reference to a `struct S`.
 
 
-## Deep Immutability (proposal)
-
 ```cpp
-struct Bar { int x; };
-Bar  b  = new Bar; /* b = new Bar; *//* b.x++; */ // immutable & deep immutable
-Bar !b  = new Bar;    b = new Bar;      b.x++;    //   mutable & deep   mutable
-val  b  = new Bar; /* b = new Bar; *//* b.x++; */ // immutable & deep immutable
-var  b  = new Bar;    b = new Bar;      b.x++;    //   mutable & inherits deep mutability from RHS
+struct Bar { int x; }
+Bar !bar = new Bar;
+bar.x = 3;         // Ok, bar is mutable
+
+struct Foo { Bar !bar; int y; }
+Foo !foo = new Foo;
+foo.bar = barl     // Ok bar is mutable
+foo.bar.x++;       // Ok foo and foo.bar and foo.bar.x are all mutable
+
+val xfoo = foo;    // Throw away mutability
+xfoo.bar.x++;      // Error, cannot mutate through xfoo
+
+print(xfoo.bar.x); // Ok to read through xfoo, prints 4
+foo.bar.x++;       // Bumps to 5
+print(xfoo.bar.x); // Ok to read through xfoo, prints 5
 ```
 
 
+
+
 ## Trinary
 
 Allow `pred ? e_true : e_false`.  Also allow `pred ? e_true`, where the false result
@@ -138,7 +128,6 @@ return i; // ERROR: Undefined name 'i'
 finalize final
 oper-assign +=, -=
 TODO
-for-interator
 switch
 
 
diff --git a/chapter17/src/main/java/com/seaofnodes/simple/Parser.java b/chapter17/src/main/java/com/seaofnodes/simple/Parser.java
index e2f8ad32..09ea21ac 100644
--- a/chapter17/src/main/java/com/seaofnodes/simple/Parser.java
+++ b/chapter17/src/main/java/com/seaofnodes/simple/Parser.java
@@ -395,7 +395,7 @@ public class Parser {
     }
 
     // Look for an unbalanced `)`, skipping balanced
-    private void skipAsgn() {
+    private Type skipAsgn() {
         int paren=0;
         while( true )
             // Next X char handles skipping complex comments
@@ -403,10 +403,8 @@ public class Parser {
             case Character.MAX_VALUE:
                 throw Utils.TODO();
             case ')':
-                if( --paren<0 ) {
-                    pos(pos()-1); // Leave the `)` behind
-                    return;
-                }
+                if( --paren<0 )
+                    return posT(pos()-1); // Leave the `)` behind
                 break;
             case '(': paren ++; break;
             default: break;
@@ -521,10 +519,12 @@ public class Parser {
         boolean hasType = t!=null && t!=Type.TOP && t!=Type.BOTTOM; // User written type
         boolean hasExpr = !(peek(';') || peek(','));   // Expression follows
         if( hasExpr && !match("=") )                   // Something else
-            { pos(old);  return parseExpression(); }
-        if( !hasExpr && t==Type.BOTTOM )  throw errorSyntax("expression");
+            { pos(old); return parseExpression(); }
+        if( !hasExpr && !hasType )  throw errorSyntax("expression");
         Node expr = hasExpr ? parseExpression() : con(t.makeInit());
-        boolean xfinal = t==Type.TOP || (hasType && hasExpr && !hasBang && _scope.inCon());
+
+        // Final if no Bang AND TMP; primitives are not-final by default
+        boolean xfinal = !hasBang && (t instanceof TypeMemPtr) && hasExpr;
 
         // Defining a new variable vs updating an old one
         if( t==null ) {
@@ -538,11 +538,19 @@ public class Parser {
             if( _scope.in(def._idx)._type!=Type.TOP && def._final && !_scope.inCon() )
                 throw error("Cannot reassign final '"+name+"'");
             t = def.type(); // Declared field type
-        } else if( !hasType && hasExpr ) {
-            // Discovered type from expression
+
+        } else if( !hasType && hasExpr ) { // Either 'val' or 'var'
+            // Either 'var' or 'val'; type comes from expression
+            xfinal = t==Type.TOP; // xfinal === 'val'; otherwise 'var'
             t = expr._type.glb();
         }
 
+        // Final is deep on ptrs
+        if( xfinal && t instanceof TypeMemPtr tmp ) {
+            t = tmp.makeRO();
+            expr = peep(new ReadOnlyNode(expr));
+        }
+
         // Auto-widen int to float
         if( expr._type instanceof TypeInteger && t instanceof TypeFloat )
             expr = peep(new ToFloatNode(expr));
@@ -550,15 +558,13 @@ public class Parser {
         expr = zsMask(expr,t);
         // Auto-deepen forward ref types
         Type e = expr._type;
-        if( e instanceof TypeMemPtr tmp && tmp._obj._fields==null )
+        if( e instanceof TypeMemPtr tmp && tmp.isFRef() )
             e = TYPES.get(tmp._obj._name);
-        // "var" and "val" use TOP and BOTTOM - but just take the expression type.
-        if( t==Type.TOP || t==Type.BOTTOM )
-            t = expr._type.glb();
         // Type is sane
         if( !e.isa(t) )
             throw error("Type " + e.str() + " is not of declared type " + t.str());
 
+        // Define a new name, vs update an old one
         if( isDecl ) {
             if( !_scope.define(name,t,xfinal,expr) )
                 throw error("Redefining name '" + name + "'");
@@ -612,7 +618,7 @@ public class Parser {
         if (_xScopes.size() > 1) throw errorSyntax("struct declarations can only appear in top level scope");
         String typeName = requireId();
         Type t = TYPES.get(typeName);
-        if( t!=null && !(t instanceof TypeMemPtr tmp && tmp._obj._fields==null) )
+        if( t!=null && !(t instanceof TypeMemPtr tmp && tmp.isFRef() ) )
             throw errorSyntax("struct '" + typeName + "' cannot be redefined");
 
         // A Block scope parse, and inspect the scope afterward for fields.
@@ -646,6 +652,8 @@ public class Parser {
     // 'id' which the caller must parse.  This lets us distinguish forward ref
     // types (which ARE valid here) from local vars in an (optional) forward
     // ref type position.
+
+    // t = int|i8|i16|i32|i64|u8|u16|u32|u64|byte|bool | flt|f32|f64 | val | var | struct[?]
     private Type type() {
         int old1 = pos();
         String tname = _lexer.matchId();
@@ -654,7 +662,7 @@ public class Parser {
         Type t0 = TYPES.get(tname);
         // No new types as keywords
         if( t0 == null && KEYWORDS.contains(tname) )
-            { pos(old1); return null; }
+            return posT(old1);
         Type t1 = t0 == null ? TypeMemPtr.make(TypeStruct.makeFRef(tname)) : t0; // Null: assume a forward ref type
         // Nest arrays and '?' as needed
         while( true ) {
@@ -680,10 +688,8 @@ public class Parser {
         int old2 = pos();
         String id = _lexer.matchId();
         pos(old2);              // Reset lexer to reparse
-        if( id==null ) {
-            pos(old1);          // Reset lexer to reparse
-            return null;        // Not a type
-        }
+        if( id==null )
+            return posT(old1);  // Reset lexer to reparse
         // Yes a forward ref, so declare it
         TYPES.put(tname,t1);
         return t1;
@@ -853,7 +859,7 @@ public class Parser {
                 if( n != null ) {
                     if( n._final )
                         throw error("Cannot reassign final '"+n._name+"'");
-                    Node expr = peep(new AddNode(_scope.in(n),con(delta)));
+                    Node expr = zsMask(peep(new AddNode(_scope.in(n),con(delta))),n.type());
                     _scope.update(n,expr);
                     return expr;
                 }
@@ -887,7 +893,7 @@ public class Parser {
         if( matchx("++") || matchx("--") ) {
             if( n._final )
                 throw error("Cannot reassign final '"+n._name+"'");
-            _scope.update(n,peep(new AddNode(rvalue,con( _lexer.peek(-1)=='+' ? 1 : -1))));
+            _scope.update(n,zsMask(peep(new AddNode(rvalue,con( _lexer.peek(-1)=='+' ? 1 : -1))),n.type()));
         }
         return rvalue.unkeep();
     }
@@ -1025,7 +1031,7 @@ public class Parser {
 
         // Sanity check field name for existing
         TypeMemPtr tmp = (TypeMemPtr)TYPES.get(ptr._obj._name);
-        if( tmp == null ) throw error("Accessing unknown field '" + name + "' from '" + ptr.str() + "'");
+        if( tmp == null ) throw error("Accessing unknown field '" + name + "' from '" + ptr + "'");
         TypeStruct base = tmp._obj;
         int fidx = base.find(name);
         if( fidx == -1 ) throw error("Accessing unknown field '" + name + "' from '" + ptr.str() + "'");
@@ -1053,28 +1059,28 @@ public class Parser {
             return val.unkeep();        // "obj.a = expr" returns the expression while updating memory
         }
 
-        Node load = new LoadNode(name, f._alias, f._type.glb(), memAlias(f._alias), expr, off);
+        Node load = new LoadNode(name, f._alias, f._type.glb(), memAlias(f._alias), expr.keep(), off);
         // Arrays include control, as a proxy for a safety range check
         // Structs don't need this; they only need a NPE check which is
         // done via the type system.
         if( base.isAry() ) load.setDef(0,ctrl());
         load = peep(load);
 
-        // ary[idx]++
+        // ary[idx]++ or ptr.fld++
         if( matchx("++") || matchx("--") ) {
             if( f._final && !f._fname.equals("[]") )
                 throw error("Cannot reassign final '"+f._fname+"'");
             Node inc = peep(new AddNode(load,con( _lexer.peek(-1)=='+' ? 1 : -1)));
             Node val = zsMask(inc,f._type);
-            Node st = new StoreNode(name, f._alias, f._type, memAlias(f._alias), expr, off, val, false);
+            Node st = new StoreNode(name, f._alias, f._type, memAlias(f._alias), expr.unkeep(), off, val, false);
             // Arrays include control, as a proxy for a safety range check.
             // Structs don't need this; they only need a NPE check which is
             // done via the type system.
             if( base.isAry() )  st.setDef(0,ctrl());
             memAlias(f._alias, peep(st));
             // And use the original loaded value as the result
-        }
-        if( off.unkeep().isDead() ) off.kill();
+        } else expr.unkill();
+        off.unkill();
 
         return parsePostfix(load);
     }
@@ -1134,6 +1140,7 @@ public class Parser {
         _lexer._position = pos;
         return old;
     }
+    private Type posT(int pos) { _lexer._position = pos; return null; }
 
     // Require and return an identifier
     private String requireId() {
diff --git a/chapter17/src/main/java/com/seaofnodes/simple/node/LoadNode.java b/chapter17/src/main/java/com/seaofnodes/simple/node/LoadNode.java
index 555b9ae4..1528f2e9 100644
--- a/chapter17/src/main/java/com/seaofnodes/simple/node/LoadNode.java
+++ b/chapter17/src/main/java/com/seaofnodes/simple/node/LoadNode.java
@@ -63,13 +63,15 @@ public class LoadNode extends MemOpNode {
         // Load-after-Store on same address, but bypassing provably unrelated
         // stores.  This is a more complex superset of the above two peeps.
         // "Provably unrelated" is really weak.
+        if( ptr instanceof ReadOnlyNode ro )
+            ptr = ro.in(1);
         Node mem = mem();
         outer:
         while( true ) {
             switch( mem ) {
             case StoreNode st:
                 if( ptr == st.ptr() && off() == st.off() )
-                    return st.val(); // Proved equal
+                    return castRO(st.val()); // Proved equal
                 // Can we prove unequal?  Offsets do not overlap?
                 if( !off()._type.join(st.off()._type).isHigh() && // Offsets overlap
                     !neverAlias(ptr,st.ptr()) )                   // And might alias
@@ -82,7 +84,7 @@ public class LoadNode extends MemOpNode {
             case ProjNode mproj:
                 if( mproj.in(0) instanceof NewNode nnn1 ) {
                     if( ptr instanceof ProjNode pproj && pproj.in(0) == mproj.in(0) )
-                        return nnn1.in(nnn1.findAlias(_alias)); // Load from New init
+                        return castRO(nnn1.in(nnn1.findAlias(_alias))); // Load from New init
                     if( !(ptr instanceof ProjNode pproj && pproj.in(0) instanceof NewNode nnn2) )
                         break outer; // Cannot tell, ptr not related to New
                     mem = nnn1.in(_alias);// Bypass unrelated New
@@ -120,10 +122,12 @@ public class LoadNode extends MemOpNode {
 
         return null;
     }
+
     private Node ld( int idx ) {
         Node mem = mem(), ptr = ptr();
         return new LoadNode(_name,_alias,_declaredType,mem.in(idx),ptr instanceof PhiNode && ptr.in(0)==mem.in(0) ? ptr.in(idx) : ptr, off()).peephole();
     }
+
     private static boolean neverAlias( Node ptr1, Node ptr2 ) {
         return ptr1.in(0) != ptr2.in(0) &&
             // Unrelated allocations
@@ -158,4 +162,11 @@ public class LoadNode extends MemOpNode {
         if( px instanceof StoreNode st1 && ptr()==st1.ptr() && off()==st1.off() ) { px.addDep(this); return true; }
         return false;
     }
+
+    // Read-Only is a deep property, and cannot be cast-away
+    private Node castRO(Node rez) {
+        if( ptr()._type.isFinal() && !rez._type.isFinal() )
+            return new ReadOnlyNode(rez).peephole();
+        return rez;
+    }
 }
diff --git a/chapter17/src/main/java/com/seaofnodes/simple/node/PhiNode.java b/chapter17/src/main/java/com/seaofnodes/simple/node/PhiNode.java
index 58ce6b47..cb266ca3 100644
--- a/chapter17/src/main/java/com/seaofnodes/simple/node/PhiNode.java
+++ b/chapter17/src/main/java/com/seaofnodes/simple/node/PhiNode.java
@@ -47,7 +47,7 @@ public class PhiNode extends Node {
         // During parsing Phis have to be computed type pessimistically.
         if( r.inProgress() ) return _declaredType;
         // Set type to local top of the starting type
-        Type t = _declaredType.glb().dual();
+        Type t = _declaredType.lub();
         for (int i = 1; i < nIns(); i++)
             // If the region's control input is live, add this as a dependency
             // to the control because we can be peeped should it become dead.
diff --git a/chapter17/src/main/java/com/seaofnodes/simple/node/ReadOnlyNode.java b/chapter17/src/main/java/com/seaofnodes/simple/node/ReadOnlyNode.java
new file mode 100644
index 00000000..fddcaa67
--- /dev/null
+++ b/chapter17/src/main/java/com/seaofnodes/simple/node/ReadOnlyNode.java
@@ -0,0 +1,29 @@
+package com.seaofnodes.simple.node;
+
+import com.seaofnodes.simple.type.Type;
+import com.seaofnodes.simple.type.TypeMemPtr;
+
+import java.util.BitSet;
+
+/**
+ * Cast a pointer to read-only
+ */
+public class ReadOnlyNode extends Node {
+    public ReadOnlyNode( Node n ) { super(null,n); }
+    @Override public String label() { return "ReadOnly"; }
+
+    @Override
+    StringBuilder _print1(StringBuilder sb, BitSet visited) {
+        return in(1)._print0(sb.append("(const)"),visited);
+    }
+    @Override public Type compute() {
+        Type t = in(1)._type;
+        return t instanceof TypeMemPtr tmp ? tmp.makeRO() : t;
+    }
+
+    @Override public Node idealize() {
+        if( in(1)._type instanceof TypeMemPtr tmp && tmp.isFinal() )
+            return in(1);
+        return null;
+    }
+}
diff --git a/chapter17/src/main/java/com/seaofnodes/simple/node/ScopeMinNode.java b/chapter17/src/main/java/com/seaofnodes/simple/node/ScopeMinNode.java
index c51349fa..1cff690c 100644
--- a/chapter17/src/main/java/com/seaofnodes/simple/node/ScopeMinNode.java
+++ b/chapter17/src/main/java/com/seaofnodes/simple/node/ScopeMinNode.java
@@ -29,12 +29,12 @@ public class ScopeMinNode extends Node {
             Type def = Parser.TYPES.get(((TypeMemPtr)_type)._obj._name);
             return (_type=_type.meet(def));
         }
-        public Type glb() {
+        public Type lazyGLB() {
             Type t = type();
             return t instanceof TypeMemPtr ? t : t.glb();
         }
         @Override public String toString() {
-            return _type.toString()+(_final ? " !": " ")+_name;
+            return _type.toString()+(_final ? " ": " !")+_name;
         }
     }
 
diff --git a/chapter17/src/main/java/com/seaofnodes/simple/node/ScopeNode.java b/chapter17/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
index 5a735515..bfb6eed6 100644
--- a/chapter17/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
+++ b/chapter17/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
@@ -167,7 +167,7 @@ public class ScopeNode extends ScopeMinNode {
                 // Set real Phi in the loop head
                 // The phi takes its one input (no backedge yet) from a recursive
                 // lookup, which might have insert a Phi in every loop nest.
-                : loop.setDef(v._idx,new PhiNode(v._name, v.glb(), loop.ctrl(), loop.in(loop.update(v,null)._idx),null).peephole());
+                : loop.setDef(v._idx,new PhiNode(v._name, v.lazyGLB(), loop.ctrl(), loop.in(loop.update(v,null)._idx),null).peephole());
             setDef(v._idx,old);
         }
         if( st!=null ) setDef(v._idx,st); // Set new value
diff --git a/chapter17/src/main/java/com/seaofnodes/simple/node/StoreNode.java b/chapter17/src/main/java/com/seaofnodes/simple/node/StoreNode.java
index 8e3e6693..62b49a1a 100644
--- a/chapter17/src/main/java/com/seaofnodes/simple/node/StoreNode.java
+++ b/chapter17/src/main/java/com/seaofnodes/simple/node/StoreNode.java
@@ -114,6 +114,9 @@ public class StoreNode extends MemOpNode {
     String err() {
         String err = super.err();
         if( err != null ) return err;
+        TypeMemPtr tmp = (TypeMemPtr)ptr()._type;
+        if( tmp._obj.field(_name)._final )
+            return "Cannot modify final field '"+_name+"'";
         Type t = val()._type;
         return _init || t.isa(_declaredType) ? null : "Cannot store "+t+" into field "+_declaredType+" "+_name;
     }
diff --git a/chapter17/src/main/java/com/seaofnodes/simple/type/Field.java b/chapter17/src/main/java/com/seaofnodes/simple/type/Field.java
index e2c0aa59..349c1fb0 100644
--- a/chapter17/src/main/java/com/seaofnodes/simple/type/Field.java
+++ b/chapter17/src/main/java/com/seaofnodes/simple/type/Field.java
@@ -31,8 +31,10 @@ public class Field extends Type {
         return new Field(fname,type,alias,xfinal).intern();
     }
     public Field makeFrom( Type type ) {
-        return new Field(_fname,type,_alias,_final).intern();
+        return type == _type ? this : new Field(_fname,type,_alias,_final).intern();
     }
+    @Override public Field makeRO() { return _final ? this : make(_fname,_type.makeRO(),_alias,true);  }
+    @Override public boolean isFinal() { return _final && _type.isFinal(); }
 
     public static final Field TEST = make("test",TypeInteger.ZERO,-2,false);
     public static void gather(ArrayList<Type> ts) { ts.add(TEST); }
@@ -41,16 +43,20 @@ public class Field extends Type {
         Field fld = (Field)that; // Invariant
         assert _fname.equals(fld._fname);
         assert _alias==fld._alias;
-        assert _final==fld._final;
-        return make(_fname,_type.meet(fld._type),_alias,_final);
+        return make(_fname,_type.meet(fld._type),_alias,_final | fld._final);
     }
 
     @Override
-    public Field dual() { return make(_fname,_type.dual(),_alias,_final); }
+    public Field dual() { return make(_fname,_type.dual(),_alias,!_final); }
 
-    @Override
-    public Field glb() {
-        return make(_fname,_type.glb(),_alias,_final);
+    @Override public Field glb() {
+        Type glb = _type.glb();
+        return glb==_type ? this : make(_fname,glb,_alias,_final);
+    }
+
+    @Override public Field lub() {
+        Type lub = _type.lub();
+        return lub==_type ? this : make(_fname,lub,_alias,_final);
     }
 
     // Override in subclasses
@@ -64,7 +70,7 @@ public class Field extends Type {
 
     @Override
     public StringBuilder print( StringBuilder sb ) {
-        return _type.print(sb.append(_final?"!":"").append(_fname).append(":").append(_alias).append(" : "));
+        return _type.print(sb.append(_final?"":"!").append(_fname).append(":").append(_alias).append(" : "));
     }
 
     @Override public String str() { return _fname; }
diff --git a/chapter17/src/main/java/com/seaofnodes/simple/type/Type.java b/chapter17/src/main/java/com/seaofnodes/simple/type/Type.java
index e454f48f..84b0119a 100644
--- a/chapter17/src/main/java/com/seaofnodes/simple/type/Type.java
+++ b/chapter17/src/main/java/com/seaofnodes/simple/type/Type.java
@@ -81,9 +81,6 @@ public class Type {
      */
     public StringBuilder typeName( StringBuilder sb) { return print(sb); }
 
-    public Type makeInit() { return null; }
-    public Type makeZero() { return null; }
-    public Type nonZero() { return glb(); }
 
     // ----------------------------------------------------------
 
@@ -175,14 +172,29 @@ public class Type {
     // True if this "isa" t; e.g. 17 isa TypeInteger.BOT
     public boolean isa( Type t ) { return meet(t)==t; }
 
-    /**
-     * Compute greatest lower bound in the lattice
-     */
-    public Type glb() { return _type==TCTRL ? XCONTROL : BOTTOM; }
+    /** Compute greatest lower bound in the lattice */
+    public Type glb() { return _type==TCTRL ? CONTROL : BOTTOM; }
+    /** Compute least upper bound in the lattice */
+    public Type lub() { return _type==TCTRL ? XCONTROL : TOP; }
+
+    // Make an initial/default version of this type.  Typically, 0 for integers
+    // and null for nullable pointers.
+    public Type makeInit() { return null; }
+    // Make a zero version of this type, 0 for integers and null for pointers.
+    public Type makeZero() { return null; }
+    // Make a non-zero version of this type, if possible.  Integers attempt to
+    // exclude zero from their range and pointers become not-null.
+    public Type nonZero() { return glb(); }
 
     // Is forward-reference
     public boolean isFRef() { return false; }
 
+    // All reachable struct Fields are final
+    public boolean isFinal() { return true; }
+
+    // Make all reachable struct Fields final
+    public Type makeRO() { return this; }
+
     // ----------------------------------------------------------
 
     // Size in bits to hold an instance of this type.
diff --git a/chapter17/src/main/java/com/seaofnodes/simple/type/TypeFloat.java b/chapter17/src/main/java/com/seaofnodes/simple/type/TypeFloat.java
index 37f6ce71..f85abbed 100644
--- a/chapter17/src/main/java/com/seaofnodes/simple/type/TypeFloat.java
+++ b/chapter17/src/main/java/com/seaofnodes/simple/type/TypeFloat.java
@@ -104,9 +104,8 @@ public class TypeFloat extends Type {
         return make((byte)-_sz,0);
     }
 
-    @Override
-    public Type glb() { return BOT; }
-
+    @Override public Type glb() { return BOT; }
+    @Override public Type lub() { return TOP; }
     @Override public TypeFloat makeInit() { return ZERO; }
     @Override public TypeFloat makeZero() { return ZERO; }
 
diff --git a/chapter17/src/main/java/com/seaofnodes/simple/type/TypeInteger.java b/chapter17/src/main/java/com/seaofnodes/simple/type/TypeInteger.java
index 06848dc7..b2979d7b 100644
--- a/chapter17/src/main/java/com/seaofnodes/simple/type/TypeInteger.java
+++ b/chapter17/src/main/java/com/seaofnodes/simple/type/TypeInteger.java
@@ -115,6 +115,7 @@ public class TypeInteger extends Type {
 
     @Override public TypeInteger dual() { return make(_max,_min); }
     @Override public TypeInteger glb() { return BOT; }
+    @Override public TypeInteger lub() { return TOP; }
     @Override public TypeInteger makeInit() { return ZERO; }
     @Override public TypeInteger makeZero() { return ZERO; }
     @Override public TypeInteger nonZero() {
diff --git a/chapter17/src/main/java/com/seaofnodes/simple/type/TypeMem.java b/chapter17/src/main/java/com/seaofnodes/simple/type/TypeMem.java
index 32491a11..33c6a8ce 100644
--- a/chapter17/src/main/java/com/seaofnodes/simple/type/TypeMem.java
+++ b/chapter17/src/main/java/com/seaofnodes/simple/type/TypeMem.java
@@ -21,7 +21,7 @@ public class TypeMem extends Type {
     private TypeMem(int alias, Type t) { super(TMEM); _alias = alias; _t = t; }
 
     public static TypeMem make(int alias, Type t) { return new TypeMem(alias,t).intern(); }
-    public static final TypeMem TOP = make( 0, Type.TOP);
+    public static final TypeMem TOP = make(-1, Type.TOP   );
     public static final TypeMem BOT = make(-1, Type.BOTTOM);
 
     public static void gather(ArrayList<Type> ts) { ts.add(make(1,TypeInteger.ZERO)); ts.add(BOT); }
@@ -40,13 +40,11 @@ public class TypeMem extends Type {
 
     @Override
     public Type dual() {
-        if( _alias== 0 ) return BOT;
-        if( _alias==-1 ) return TOP;
         return make(_alias,_t.dual());
     }
 
-    @Override
-    public Type glb() { return make(_alias,_t.glb()); }
+    @Override public Type glb() { return make(_alias,_t.glb()); }
+    @Override public Type lub() { return make(_alias,_t.lub()); }
 
     @Override
     int hash() { return 9876543 + _alias + _t.hashCode(); }
diff --git a/chapter17/src/main/java/com/seaofnodes/simple/type/TypeMemPtr.java b/chapter17/src/main/java/com/seaofnodes/simple/type/TypeMemPtr.java
index cc4b9fcd..08e7db1c 100644
--- a/chapter17/src/main/java/com/seaofnodes/simple/type/TypeMemPtr.java
+++ b/chapter17/src/main/java/com/seaofnodes/simple/type/TypeMemPtr.java
@@ -33,7 +33,9 @@ public class TypeMemPtr extends Type {
     }
     public static TypeMemPtr make(TypeStruct obj, boolean nil) { return new TypeMemPtr(obj, nil).intern(); }
     public static TypeMemPtr make(TypeStruct obj) { return make(obj, false); }
-    public TypeMemPtr makeFrom(TypeStruct obj) { return make(obj, _nil); }
+    public TypeMemPtr makeFrom(TypeStruct obj) { return obj==_obj ? this : make(obj, _nil); }
+    @Override public TypeMemPtr makeRO() { return makeFrom(_obj.makeRO()); }
+    @Override public boolean isFinal() { return _obj.isFinal(); }
 
     // An abstract pointer, pointing to either a Struct or an Array.
     // Can also be null or not.
@@ -55,8 +57,8 @@ public class TypeMemPtr extends Type {
     @Override
     public TypeMemPtr dual() { return TypeMemPtr.make(_obj.dual(), !_nil); }
 
-    @Override
-    public TypeMemPtr glb() { return make(_obj.glb(),true); }
+    @Override public TypeMemPtr glb() { return make(_obj.glb(),true ); }
+    @Override public TypeMemPtr lub() { return make(_obj.lub(),false); }
     // Is forward-reference
     @Override public boolean isFRef() { return _obj.isFRef(); }
     @Override public Type makeInit() { return _nil ? NULLPTR : Type.TOP; }
diff --git a/chapter17/src/main/java/com/seaofnodes/simple/type/TypeStruct.java b/chapter17/src/main/java/com/seaofnodes/simple/type/TypeStruct.java
index 39011ce2..b3dbcad0 100644
--- a/chapter17/src/main/java/com/seaofnodes/simple/type/TypeStruct.java
+++ b/chapter17/src/main/java/com/seaofnodes/simple/type/TypeStruct.java
@@ -37,20 +37,28 @@ public class TypeStruct extends Type {
     public static final TypeStruct TEST = make("test",new Field[]{Field.TEST});
     // Forward-ref version
     public static TypeStruct makeFRef(String name) { return make(name, (Field[])null); }
+    // Make a read-only version
+    @Override public TypeStruct makeRO() {
+        if( isFinal() ) return this;
+        Field[] flds = new Field[_fields.length];
+        for( int i=0; i<flds.length; i++ )
+            flds[i] = _fields[i].makeRO();
+        return make(_name,flds);
+    }
 
     // Array
     public static TypeStruct makeAry(TypeInteger len, int lenAlias, Type body, int bodyAlias) {
         assert !(body instanceof TypeMemPtr tmp && !tmp._nil);
         return make("[" + body.str() + "]",
-                    Field.make("#" ,len , lenAlias,true),
-                    Field.make("[]",body,bodyAlias,true));
+                    Field.make("#" ,len , lenAlias,true ),
+                    Field.make("[]",body,bodyAlias,false));
     }
 
     // A pair of self-cyclic types
     private static final TypeStruct S1F = makeFRef("S1");
     private static final TypeStruct S2F = makeFRef("S2");
-    public  static final TypeStruct S1  = make("S1", new Field[]{ Field.make("a", TypeInteger.BOT, -1, false), Field.make("s2",TypeMemPtr.make(S2F,false),-2, false) });
-    private static final TypeStruct S2  = make("S2", new Field[]{ Field.make("b", TypeFloat  .BOT, -3, false), Field.make("s1",TypeMemPtr.make(S1F,false),-4, false) });
+    public  static final TypeStruct S1  = make("S1", Field.make("a", TypeInteger.BOT, -1, false), Field.make("s2",TypeMemPtr.make(S2F,false),-2, false) );
+    private static final TypeStruct S2  = make("S2", Field.make("b", TypeFloat  .BOT, -3, false), Field.make("s1",TypeMemPtr.make(S1F,false),-4, false) );
 
     private static final TypeStruct ARY = makeAry(TypeInteger.BOT,-1,TypeFloat.BOT,-2);
 
@@ -63,6 +71,10 @@ public class TypeStruct extends Type {
                 return i;
         return -1;
     }
+    public Field field(String fname) {
+        int idx = find(fname);
+        return idx== -1 ? null : _fields[idx];
+    }
 
     // Find field index by alias
     public int findAlias( int alias ) {
@@ -115,8 +127,7 @@ public class TypeStruct extends Type {
     }
 
     // Keeps the same struct, but lower-bounds all fields.
-    @Override
-    public TypeStruct glb() {
+    @Override public TypeStruct glb() {
         if( _glb() ) return this;
         // Need to glb each field
         Field[] flds = new Field[_fields.length];
@@ -131,9 +142,33 @@ public class TypeStruct extends Type {
                  return false;
         return true;
     }
+    @Override public TypeStruct lub() {
+        if( _lub() ) return this;
+        // Need to glb each field
+        Field[] flds = new Field[_fields.length];
+        for( int i=0; i<_fields.length; i++ )
+            flds[i] = _fields[i].lub();
+        return make(_name,flds);
+    }
+    private boolean _lub() {
+        if( _fields!=null )
+          for( Field f : _fields )
+              if( f.lub() != f )
+                 return false;
+        return true;
+    }
+
 
     // Is forward-reference
     @Override public boolean isFRef() { return _fields==null; }
+    // All fields are final
+    @Override public boolean isFinal() {
+        if( _fields==null ) return true;
+        for( Field fld : _fields )
+            if( !fld.isFinal() )
+                return false;
+        return true;
+    }
 
     @Override
     boolean eq(Type t) {
@@ -164,10 +199,8 @@ public class TypeStruct extends Type {
         sb.append(_name);
         if( _fields == null ) return sb; // Forward reference struct, just print the name
         sb.append(" {");
-        for( Field f : _fields ) {
-            sb.append("  ").append(f._fname).append(" :");
-            f._type.print(sb).append("; ");
-        }
+        for( Field f : _fields )
+            f._type.print(sb).append(f._final ? " " : " !").append(f._fname).append("; ");
         return sb.append("}");
     }
 
diff --git a/chapter17/src/test/java/com/seaofnodes/simple/BrainFuckTest.java b/chapter17/src/test/java/com/seaofnodes/simple/BrainFuckTest.java
index 4833483b..88d79215 100644
--- a/chapter17/src/test/java/com/seaofnodes/simple/BrainFuckTest.java
+++ b/chapter17/src/test/java/com/seaofnodes/simple/BrainFuckTest.java
@@ -15,7 +15,7 @@ public class BrainFuckTest {
     @Test
     public void testBrainfuck() {
         var program = "++++++++[>++++[>++>+++>+++>+<<<<-]>+>+>->>+[<]<-]>>.>---.+++++++..+++.>>.<-.<.+++.------.--------.>>+.>++.".getBytes(StandardCharsets.UTF_8);
-        var encoded = new StringBuilder("u8[] program = new u8[").append(program.length).append("];");
+        var encoded = new StringBuilder("u8[] !program = new u8[").append(program.length).append("];");
         for (int i = 0; i < program.length; i++) {
             int value = program[i] & 0xFF;
             encoded.append("program[").append(i).append("] = ").append(value).append(";");
@@ -25,7 +25,7 @@ public class BrainFuckTest {
                 encoded + """
 var d = 0;
 u8[] !output = new u8[0];
-val data = new u8[100];
+u8[] !data = new u8[100];
 
 for( int pc = 0; pc < program#; pc++ ) {
     var command = program[pc];
diff --git a/chapter17/src/test/java/com/seaofnodes/simple/Chapter10Test.java b/chapter17/src/test/java/com/seaofnodes/simple/Chapter10Test.java
index 44d71b1e..d41c83d8 100644
--- a/chapter17/src/test/java/com/seaofnodes/simple/Chapter10Test.java
+++ b/chapter17/src/test/java/com/seaofnodes/simple/Chapter10Test.java
@@ -44,7 +44,7 @@ struct Foo {
     int x;
 };
 Foo? foo = null;
-Bar bar = new Bar;
+Bar !bar = new Bar;
 bar.a = 1;
 bar.a = 2;
 return bar.a;
@@ -57,7 +57,7 @@ return bar.a;
     public void testExample() {
         Parser parser = new Parser("""
 struct Vector2D { int x; int y; };
-Vector2D v = new Vector2D;
+Vector2D !v = new Vector2D;
 v.x = 1;
 if (arg)
     v.y = 2;
@@ -96,7 +96,7 @@ arg=0+new s0.0;
     public void testLoop() {
         Parser parser = new Parser("""
 struct Bar { int a; };
-Bar bar = new Bar;
+Bar !bar = new Bar;
 while (arg) {
     bar.a = bar.a + 2;
     arg = arg + 1;
@@ -111,7 +111,7 @@ return bar.a;
     public void testIf() {
         Parser parser = new Parser("""
 struct Bar { int a; };
-Bar bar = new Bar;
+Bar !bar = new Bar;
 if (arg) bar = null;
 bar.a = 1;
 return bar.a;
@@ -124,7 +124,7 @@ return bar.a;
     public void testIf2() {
         Parser parser = new Parser("""
 struct Bar { int a; };
-Bar? bar = null;
+Bar? !bar = null;
 if (arg) bar = new Bar;
 bar.a = 1;
 return bar.a;
@@ -150,7 +150,7 @@ return bar.a;
     public void testIfOrNull() {
         Parser parser = new Parser("""
 struct Bar { int a; };
-Bar? bar = new Bar;
+Bar? !bar = new Bar;
 if (arg) bar = null;
 if( bar ) bar.a = 1;
 return bar;
@@ -164,7 +164,7 @@ return bar;
         Parser parser = new Parser(
 """
 struct Bar { int a; };
-Bar? bar = new Bar;
+Bar? !bar = new Bar;
 if (arg) bar = null;
 int rez = 3;
 if( !bar ) rez=4;
@@ -179,7 +179,7 @@ return rez;
     public void testWhileWithNullInside() {
         Parser parser = new Parser("""
 struct s0 {int v0;};
-s0? v0 = new s0;
+s0? !v0 = new s0;
 int ret = 0;
 while(arg) {
     ret = v0.v0;
@@ -216,7 +216,7 @@ struct Iter {
     int x;
     int len;
 };
-Iter i = new Iter;
+Iter !i = new Iter;
 i.len = arg;
 int sum=0;
 while( i.x < i.len ) {
@@ -234,9 +234,9 @@ return sum;
     public void test1() {
         Parser parser = new Parser("""
 struct s0 {int v0;};
-s0 ret = new s0;
+s0 !ret = new s0;
 while(arg) {
-    s0 v0 = new s0;
+    s0 !v0 = new s0;
     v0.v0 = arg;
     arg = arg-1;
     if (arg==5) ret=v0;
@@ -252,8 +252,8 @@ return ret;
     public void test2() {
         Parser parser = new Parser("""
 struct s0 {int v0;};
-s0 ret = new s0;
-s0 v0 = new s0;
+s0 !ret = new s0;
+s0 !v0 = new s0;
 while(arg) {
     v0.v0 = arg;
     arg = arg-1;
@@ -271,9 +271,9 @@ return ret;
     public void test3() {
         Parser parser = new Parser("""
 struct s0 {int v0;};
-s0 ret = new s0;
+s0 !ret = new s0;
 while(arg < 10) {
-    s0 v0 = new s0;
+    s0 !v0 = new s0;
     if (arg == 5) ret=v0;
     arg = arg + 1;
 }
@@ -350,7 +350,7 @@ s0 v1 = v0;
 return v1;
     """);
         StopNode stop = parser.parse().iterate();
-        assertEquals("return s0;", stop.toString());
+        assertEquals("return (const)s0;", stop.toString());
     }
 
 
diff --git a/chapter17/src/test/java/com/seaofnodes/simple/Chapter11Test.java b/chapter17/src/test/java/com/seaofnodes/simple/Chapter11Test.java
index ddcc9e63..79c0622c 100644
--- a/chapter17/src/test/java/com/seaofnodes/simple/Chapter11Test.java
+++ b/chapter17/src/test/java/com/seaofnodes/simple/Chapter11Test.java
@@ -338,7 +338,7 @@ return primeCount;
         Parser parser = new Parser(
 """
 struct S { int f; };
-S v=new S;
+S !v=new S;
 v.f = 2;
 int i=new S.f;
 i=v.f;
@@ -356,9 +356,9 @@ return i;
         Parser parser = new Parser(
 """
 struct S { int f; };
-S v = new S;
+S !v = new S;
 v.f = arg;
-S t = new S;
+S !t = new S;
 int i = 0;
 if (arg) {
     if (arg+1) v = t;
@@ -377,7 +377,7 @@ return i;
         Parser parser = new Parser(
 """
 struct S { int f; };
-S v0 = new S;
+S !v0 = new S;
 S? v1;
 if (arg) v1 = new S;
 if (v1) {
@@ -397,7 +397,7 @@ return v0;
         Parser parser = new Parser(
 """
 struct S { int f; };
-S v = new S;
+S !v = new S;
 v.f = arg;
 S t = new S;
 int i = v.f;
@@ -414,7 +414,7 @@ return i;
         Parser parser = new Parser(
 """
 struct S { int f; };
-S v = new S;
+S !v = new S;
 while(1) {
     while(arg+1) { arg=arg-1; }
     if (arg) break;
@@ -431,7 +431,7 @@ return v;
         Parser parser = new Parser(
 """
 struct s { int v; };
-s ptr=new s;
+s !ptr=new s;
 while( -arg )
   ptr = new s;
 while(1)
@@ -446,7 +446,7 @@ while(1)
         Parser parser = new Parser(
 """
 struct S { int f; };
-S v = new S;
+S !v = new S;
 S t = new S;
 int i = v.f;
 while (arg) {
@@ -464,7 +464,7 @@ return i;
         Parser parser = new Parser(
 """
 struct S { int f; };
-S v = new S;
+S !v = new S;
 S t = new S;
 while(arg) {
     arg=arg-1;
@@ -484,7 +484,7 @@ return arg;
         Parser parser = new Parser(
 """
 struct S { int f; };
-S v = new S;
+S !v = new S;
 S t = new S;
 if (arg) {
     v.f=2;
@@ -502,7 +502,7 @@ return v;
         Parser parser = new Parser(
 """
 struct S { int f; };
-S v = new S;
+S !v = new S;
 int i = arg;
 while (arg > 0) {
     int j = i/3;
diff --git a/chapter17/src/test/java/com/seaofnodes/simple/Chapter13Test.java b/chapter17/src/test/java/com/seaofnodes/simple/Chapter13Test.java
index aa659147..46eac241 100644
--- a/chapter17/src/test/java/com/seaofnodes/simple/Chapter13Test.java
+++ b/chapter17/src/test/java/com/seaofnodes/simple/Chapter13Test.java
@@ -23,9 +23,9 @@ return 3.14;
         Parser parser = new Parser(
 """
 struct LLI { LLI? next; int i; };
-LLI? head = null;
+LLI? !head = null;
 while( arg ) {
-    LLI x = new LLI;
+    LLI !x = new LLI;
     x.next = head;
     x.i = arg;
     head = x;
@@ -42,9 +42,9 @@ return head.next.i;
         Parser parser = new Parser(
 """
 struct LLI { LLI? next; int i; };
-LLI? head = null;
+LLI? !head = null;
 while( arg ) {
-    LLI x = new LLI;
+    LLI !x = new LLI;
     x.next = head;
     x.i = arg;
     head = x;
@@ -66,9 +66,9 @@ return next.i;
 """
 struct int0 { int i; flt0? f; };
 struct flt0 { flt f; int0? i; };
-int0 i0 = new int0;
+int0 !i0 = new int0;
 i0.i = 17;
-flt0 f0 = new flt0;
+flt0 !f0 = new flt0;
 f0.f = 3.14;
 i0.f = f0;
 f0.i = i0;
@@ -100,7 +100,7 @@ N n = new N { next = new M; };
 return n.next;
 """);
         StopNode stop = parser.parse().iterate();
-        assertEquals("return M;", stop.toString());
+        assertEquals("return (const)M;", stop.toString());
     }
 
     @Test
@@ -121,7 +121,7 @@ return n.next;
         Parser parser = new Parser(
 """
 struct N { N? next; int i; };
-N n = new N;
+N !n = new N;
 n.i = 3.14;
 return n.i;
 """);
@@ -169,7 +169,7 @@ return new S1.s=new S2;
 """
 struct I {int i;};
 struct P { I? pi; };
-P p1 = new P;
+P !p1 = new P;
 P p2 = new P;
 p2.pi = new I;
 p2.pi.i = 2;
diff --git a/chapter17/src/test/java/com/seaofnodes/simple/Chapter14Test.java b/chapter17/src/test/java/com/seaofnodes/simple/Chapter14Test.java
index 01cecebb..23c82358 100644
--- a/chapter17/src/test/java/com/seaofnodes/simple/Chapter14Test.java
+++ b/chapter17/src/test/java/com/seaofnodes/simple/Chapter14Test.java
@@ -94,7 +94,7 @@ return b;
         Parser parser = new Parser(
 """
 struct Foo { u1 b; };
-Foo f = new Foo;
+Foo !f = new Foo;
 f.b = 123;
 return f.b;
 """);
diff --git a/chapter17/src/test/java/com/seaofnodes/simple/Chapter15Test.java b/chapter17/src/test/java/com/seaofnodes/simple/Chapter15Test.java
index 04958819..4e2682dc 100644
--- a/chapter17/src/test/java/com/seaofnodes/simple/Chapter15Test.java
+++ b/chapter17/src/test/java/com/seaofnodes/simple/Chapter15Test.java
@@ -25,7 +25,7 @@ return 3.14;
         Parser parser = new Parser(
 """
 struct C { C? l; };
-C c = new C;
+C !c = new C;
 c.l = c;
 return c;
 """);
@@ -39,7 +39,7 @@ return c;
         Parser parser = new Parser(
 """
 u8[] old = new u8[0];
-u8[] output = new u8[1];
+u8[] !output = new u8[1];
 int i = 0;
 while (i < old#) {
     output[i] = old[i];
@@ -82,7 +82,7 @@ return is[1];
     public void testBasic3() {
         Parser parser = new Parser(
 """
-int[] a = new int[2];
+int[] !a = new int[2];
 a[0] = 1;
 a[1] = 2;
 return a[0];
@@ -97,7 +97,7 @@ return a[0];
         Parser parser = new Parser(
 """
 struct A { int i; };
-A?[] a = new A?[2];
+A?[] !a = new A?[2];
 return a;
 """);
         StopNode stop = parser.parse().iterate();
@@ -111,10 +111,10 @@ return a;
 """
 struct S { int x; flt y; };
 // A new S
-S s = new S; s.x=99; s.y = 3.14;
+S !s = new S; s.x=99; s.y = 3.14;
 
 // Double-d array of Ss.  Fill in one row.
-S?[]?[] iss = new S?[]?[2];
+S?[]?[] !iss = new S?[]?[2];
 iss[0] = new S?[7];
 iss[0][2] = s;
 
@@ -135,14 +135,13 @@ return rez;
         assertEquals(1.2 , Evaluator.evaluate(stop, 1));
     }
 
-    @Ignore
     @Test
     public void testBasic6() {
         Parser parser = new Parser(
 """
 struct S { int x; flt y; };
 // A new S
-S s = new S; s.x=99; s.y = 3.14;
+S !s = new S; s.x=99; s.y = 3.14;
 
 // Double-d array of Ss.  Fill in one row.
 S?[]?[] iss = new S?[]?[2];
@@ -156,9 +155,8 @@ if( iss[arg] )
         rez = iss[arg][2].y;
 return rez;
 """);
-        StopNode stop = parser.parse().iterate();
-        assertEquals("return Phi(Region,Phi(Region,.y,1.2),1.2);", stop.toString());
-        assertEquals(3.14, Evaluator.evaluate(stop, 0));
+        try { parser.parse().iterate(); fail(); }
+        catch( Exception e ) { assertEquals("Might be null accessing 'y'",e.getMessage()); }
     }
 
     @Test
@@ -167,8 +165,8 @@ return rez;
 """
 // Can we define a forward-reference array?
 struct Tree { Tree?[]? _kids; };
-Tree root = new Tree;
-root._kids = new Tree?[2];
+Tree !root = new Tree;
+root._kids = new Tree?[2]; // NO BANG SO ARRAY IS OF IMMUTABLE TREES????
 root._kids[0] = new Tree;
 return root;
 """);
@@ -192,7 +190,7 @@ return 0;
     public void testRollingSum() {
         Parser parser = new Parser(
 """
-int[] ary = new int[arg];
+int[] !ary = new int[arg];
 // Fill [0,1,2,3,4,...]
 int i=0;
 while( i < ary# ) {
@@ -216,7 +214,7 @@ return ary[1] * 1000 + ary[3]; // 1 * 1000 + 6
     public void sieveOEratosthenes() {
         Parser parser = new Parser(
 """
-int[] ary = new int[arg], primes = new int[arg];
+int[] !ary = new int[arg], !primes = new int[arg];
 int nprimes = 0, p=2;
 // Find primes while p^2 < arg
 while( p*p < arg ) {
@@ -240,7 +238,7 @@ while( p < arg ) {
     p = p + 1;
 }
 // Copy/shrink the result array
-int[] rez = new int[nprimes];
+int[] !rez = new int[nprimes];
 int j = 0;
 while( j < nprimes ) {
     rez[j] = primes[j];
@@ -251,7 +249,7 @@ return rez;
         StopNode stop = parser.parse().iterate();
         assertEquals("return [int];", stop.toString());
         Evaluator.Obj obj = (Evaluator.Obj)Evaluator.evaluate(stop, 20);
-        assertEquals("[int] {  # :int;   [] :int; }",obj.struct().toString());
+        assertEquals("[int] {int #; int ![]; }",obj.struct().toString());
         long nprimes = (Long)obj.fields()[0];
         long[] primes = new long[]{2,3,5,7,11,13,17,19};
         for( int i=0; i<nprimes; i++ )
@@ -263,8 +261,8 @@ return rez;
         Parser parser = new Parser(
 """
 struct S {int i; flt f;};
-S s1 = new S;
-S s2 = new S;
+S !s1 = new S;
+S !s2 = new S;
 s2.i = 3;
 s2.f = 2.0;
 if (arg) s1 = new S;
diff --git a/chapter17/src/test/java/com/seaofnodes/simple/Chapter16Test.java b/chapter17/src/test/java/com/seaofnodes/simple/Chapter16Test.java
index d0e30a7c..533241a9 100644
--- a/chapter17/src/test/java/com/seaofnodes/simple/Chapter16Test.java
+++ b/chapter17/src/test/java/com/seaofnodes/simple/Chapter16Test.java
@@ -114,7 +114,7 @@ Point p = new Point { x=3; y=4; };
 return p;
 """);
         StopNode stop = parser.parse().iterate();
-        assertEquals("return Point;", stop.toString());
+        assertEquals("return (const)Point;", stop.toString());
         assertEquals("Obj<Point>{x=3,y=4}", Evaluator.evaluate(stop,  0).toString());
     }
 
@@ -127,7 +127,7 @@ p.x++;
 return p;
 """);
         try { parser.parse().iterate(); fail(); }
-        catch( Exception e ) { assertEquals("Cannot reassign final 'x'",e.getMessage()); }
+        catch( Exception e ) { assertEquals("Cannot modify final field 'x'",e.getMessage()); }
     }
 
     @Test
@@ -139,7 +139,7 @@ p.x++;
 return p;
 """);
         try { parser.parse().iterate(); fail(); }
-        catch( Exception e ) { assertEquals("Cannot reassign final 'x'",e.getMessage()); }
+        catch( Exception e ) { assertEquals("Cannot modify final field 'x'",e.getMessage()); }
     }
 
     @Test
@@ -170,7 +170,7 @@ return p;
     public void testStructFinal5() {
         Parser parser = new Parser("""
 struct Point { var x=3; var y=4; };
-Point p = new Point;
+Point !p = new Point;
 p.x++;
 return p;
 """);
@@ -186,7 +186,7 @@ return p;
         Parser parser = new Parser(
 """
 struct LLI { LLI? next; int i; };
-LLI? head = null;
+LLI? !head = null;
 while( arg ) {
     head = new LLI { next=head; i=arg; };
     arg = arg-1;
@@ -208,7 +208,7 @@ return next.i;
         Parser parser = new Parser(
 """
 struct LLI { LLI? next; int i; };
-LLI? head = null;
+LLI? !head = null;
 while( arg ) {
     head = new LLI {
         next=head;
diff --git a/chapter17/src/test/java/com/seaofnodes/simple/Chapter17Test.java b/chapter17/src/test/java/com/seaofnodes/simple/Chapter17Test.java
index 3773f87f..7de7d514 100644
--- a/chapter17/src/test/java/com/seaofnodes/simple/Chapter17Test.java
+++ b/chapter17/src/test/java/com/seaofnodes/simple/Chapter17Test.java
@@ -12,13 +12,24 @@ public class Chapter17Test {
     @Test
     public void testJig() {
         Parser parser = new Parser("""
-return 3.14;
+i32 v0=0;
+if(1<<arg) return 0&0;
+            int v1=0;
+            while(0<<0!=1) {
+                v1=v0;
+                    while(0<--v0)
+                        if(1&v0) continue;
+                        else arg=0;
+                if(v1) continue;
+                    arg=0;
+            }
 """);
         StopNode stop = parser.parse().iterate();
         assertEquals("return 3.14;", stop.toString());
         assertEquals(3.14, Evaluator.evaluate(stop,  0));
     }
 
+    // ---------------------------------------------------------------
     @Test
     public void testInc0() {
         Parser parser = new Parser("""
@@ -53,7 +64,7 @@ return -arg---arg--;
     @Test
     public void testInc3() {
         Parser parser = new Parser("""
-int[] xs = new int[arg];
+int[] !xs = new int[arg];
 xs[0]++;
 xs[1]++;
 return xs[0];
@@ -66,7 +77,7 @@ return xs[0];
     @Test
     public void testInc4() {
         Parser parser = new Parser("""
-u8[] xs = new u8[1];
+u8[] !xs = new u8[1];
 xs[0]--;
 return xs[0];
 """);
@@ -79,7 +90,7 @@ return xs[0];
     public void testInc5() {
         Parser parser = new Parser("""
 struct S { u16 x; };
-S s = new S;
+S !s = new S;
 s.x--;
 return s.x;
 """);
@@ -90,26 +101,134 @@ return s.x;
 
     @Test
     public void testInc6() {
-        Parser parser = new Parser("""
-return --arg;
-""");
+        Parser parser = new Parser("return --arg;");
         StopNode stop = parser.parse().iterate();
         assertEquals("return (arg+-1);", stop.toString());
         assertEquals(-1L, Evaluator.evaluate(stop, 0));
     }
 
-
     @Test
-    public void testVar0() {
-        Parser parser = new Parser("""
-var d;
-return d;
-""");
+    public void testInc7() {
+        Parser parser = new Parser("u8 x=0; return --x;");
+        StopNode stop = parser.parse().iterate();
+        assertEquals("return 255;", stop.toString());
+        assertEquals(255L, Evaluator.evaluate(stop, 0));
+    }
+
+    // ---------------------------------------------------------------
+    @Test public void testVar0() {
+        Parser parser = new Parser("var d; return d;");
         try { parser.parse().iterate(); fail(); }
         catch( Exception e ) { assertEquals("Syntax error, expected expression: ;",e.getMessage()); }
     }
 
+    @Test public void testVar1() {
+        Parser parser = new Parser("val d; return d;");
+        try { parser.parse().iterate(); fail(); }
+        catch( Exception e ) { assertEquals("Syntax error, expected expression: ;",e.getMessage()); }
+    }
+
+    @Test public void testVar2() {
+        Parser parser = new Parser("int x; x=3; x++; return x; // Ok, no initializer so x is mutable ");
+        StopNode stop = parser.parse().iterate();
+        assertEquals("return 4;", stop.toString());
+        assertEquals(4L, Evaluator.evaluate(stop, 0));
+    }
+
+    @Test public void testVar3() {
+        Parser parser = new Parser("int x=3; x++; return x; // Ok, primitive so x is mutable despite initializer ");
+        StopNode stop = parser.parse().iterate();
+        assertEquals("return 4;", stop.toString());
+        assertEquals(4L, Evaluator.evaluate(stop, 0));
+    }
+
+    @Test public void testVar4() {
+        Parser parser = new Parser("struct S{int x;}; S s; s=new S; s.x++; return s.x; // Ok, no initializer so x is mutable ");
+        StopNode stop = parser.parse().iterate();
+        assertEquals("return 1;", stop.toString());
+        assertEquals(1L, Evaluator.evaluate(stop, 0));
+    }
+
+    @Test public void testVar5() {
+        Parser parser = new Parser("struct S{int x;}; S s; s=new S{x=3;}; s.x++; return s.x; // Ok, no initializer so x is mutable ");
+        StopNode stop = parser.parse().iterate();
+        assertEquals("return 4;", stop.toString());
+        assertEquals(4L, Evaluator.evaluate(stop, 0));
+    }
+
+    @Test public void testVar6() {
+        Parser parser = new Parser("struct S{int x;}; S s=new S; s.x++; return s.x; // Error initializer so x is immutable ");
+        try { parser.parse().iterate(); fail(); }
+        catch( Exception e ) { assertEquals("Cannot modify final field 'x'",e.getMessage()); }
+    }
+
+    @Test public void testVar7() {
+        Parser parser = new Parser("struct S{int x;}; S s=new S{x=3;}; s.x++; return s.x; // Error initializer so x is immutable ");
+        try { parser.parse().iterate(); fail(); }
+        catch( Exception e ) { assertEquals("Cannot modify final field 'x'",e.getMessage()); }
+    }
+
+    @Test public void testVar8() {
+        Parser parser = new Parser("struct S{int x;}; S !s=new S; s.x++; return s.x; // Ok, has '!' so s.x is mutable ");
+        StopNode stop = parser.parse().iterate();
+        assertEquals("return 1;", stop.toString());
+        assertEquals(1L, Evaluator.evaluate(stop, 0));
+    }
+
+    @Test public void testVar9() {
+        Parser parser = new Parser("struct S{int x;}; var s=new S; s.x++; return s.x; // Ok, has var so s.x is mutable ");
+        StopNode stop = parser.parse().iterate();
+        assertEquals("return 1;", stop.toString());
+        assertEquals(1L, Evaluator.evaluate(stop, 0));
+    }
+
+    @Test public void testVar10() {
+        Parser parser = new Parser("struct S{int x;}; val s=new S; s.x++; return s.x; // Error, has val so x is immutable ");
+        try { parser.parse().iterate(); fail(); }
+        catch( Exception e ) { assertEquals("Cannot modify final field 'x'",e.getMessage()); }
+    }
+
+    @Test public void testVar11() {
+        Parser parser = new Parser("""
+struct Bar { int x; };
+Bar !bar = new Bar;
+bar.x = 3; // Ok, bar is mutable
+
+struct Foo { Bar? !bar; int y; };
+Foo !foo = new Foo { bar = bar; };
+foo.bar = bar; // Ok foo is mutable
+foo.bar.x++;   // Ok foo and foo.bar and foo.bar.x are all mutable
+
+val xfoo = foo; // Throw away mutability
+xfoo.bar.x++;   // Error, cannot mutate through xfoo
+ """);
+        try { parser.parse().iterate(); fail(); }
+        catch( Exception e ) { assertEquals("Cannot modify final field 'x'",e.getMessage()); }
+    }
+
+    @Test public void testVar12() {
+        Parser parser = new Parser("""
+struct Bar { int x; };
+Bar !bar = new Bar;
+bar.x = 3; // Ok, bar is mutable
+
+struct Foo { Bar? !bar; int y; };
+Foo !foo = new Foo;
+foo.bar = bar; // Ok bar is mutable
+foo.bar.x++;   // Ok foo and foo.bar and foo.bar.x are all mutable
+
+val xfoo = foo;        // Throw away mutability
+int x4 = xfoo.bar.x;   // Ok to read through xfoo, gets 4
+foo.bar.x++;           // Bumps to 5
+int x5 = xfoo.bar.x;   // Ok to read through xfoo, gets 5
+return x4*10+x5;
+""");
+        StopNode stop = parser.parse().iterate();
+        assertEquals("return 45;", stop.toString());
+        assertEquals(45L, Evaluator.evaluate(stop, 0));
+    }
 
+    // ---------------------------------------------------------------
     @Test
     public void testTrinary0() {
         Parser parser = new Parser("""
@@ -173,6 +292,7 @@ return b ? b.next ? b.next.x : b.x; // parses "b ? (b.next ? b.next.x : b.x) : 0
     }
 
 
+    // ---------------------------------------------------------------
     @Test
     public void testFor0() {
         Parser parser = new Parser("""
@@ -240,16 +360,18 @@ return i;
     }
 
 
+    // ---------------------------------------------------------------
     @Test
     public void testLinkedList2() {
         Parser parser = new Parser("""
 struct LLI { LLI? next; int i; };
-LLI? head = null;
+LLI? !head = null;
 while( arg-- )
     head = new LLI { next=head; i=arg; };
 int sum=0;
-for( ; head; head = head.next )
-    sum = sum + head.i;
+var ptr = head; // A read-only ptr, to be assigned from read-only next fields
+for( ; ptr; ptr = ptr.next )
+    sum = sum + ptr.i;
 return sum;
 """);
         StopNode stop = parser.parse().iterate();
@@ -261,7 +383,7 @@ return sum;
     public void sieveOfEratosthenes() {
         Parser parser = new Parser(
 """
-val ary = new bool[arg], primes = new int[arg];
+var ary = new bool[arg], primes = new int[arg];
 var nprimes=0, p=0;
 // Find primes while p^2 < arg
 for( p=2; p*p < arg; p++ ) {
@@ -278,7 +400,7 @@ for( ; p < arg; p++ )
     if( !ary[p] )
         primes[nprimes++] = p;
 // Copy/shrink the result array
-var rez = new int[nprimes];
+var !rez = new int[nprimes];
 for( int j=0; j<nprimes; j++ )
     rez[j] = primes[j];
 return rez;
@@ -286,7 +408,7 @@ return rez;
         StopNode stop = parser.parse().iterate();
         assertEquals("return [int];", stop.toString());
         Evaluator.Obj obj = (Evaluator.Obj)Evaluator.evaluate(stop, 20);
-        assertEquals("[int] {  # :int;   [] :int; }",obj.struct().toString());
+        assertEquals("[int] {int #; int ![]; }",obj.struct().toString());
         long nprimes = (Long)obj.fields()[0];
         long[] primes = new long[]{2,3,5,7,11,13,17,19};
         for( int i=0; i<nprimes; i++ )
diff --git a/chapter17/src/test/java/com/seaofnodes/simple/SchedulerTest.java b/chapter17/src/test/java/com/seaofnodes/simple/SchedulerTest.java
index 168e0ed7..c67217af 100644
--- a/chapter17/src/test/java/com/seaofnodes/simple/SchedulerTest.java
+++ b/chapter17/src/test/java/com/seaofnodes/simple/SchedulerTest.java
@@ -24,7 +24,7 @@ public class SchedulerTest {
 struct S {
     int f;
 }
-S v=new S;
+S !v=new S;
 v.f = 2;
 int i=v.f;
 if (arg) v.f=1;
@@ -42,7 +42,7 @@ return i;
 struct S {
     int f;
 };
-S v0=new S;
+S !v0=new S;
 if(arg) v0.f=1;
 return v0;
 """);
@@ -58,7 +58,7 @@ return v0;
 struct S {
     int f;
 };
-S v=new S;
+S !v=new S;
 v.f = 2;
 int i=new S.f;
 i=v.f;
diff --git a/chapter17/src/test/java/com/seaofnodes/simple/TypeTest.java b/chapter17/src/test/java/com/seaofnodes/simple/TypeTest.java
index 03b0e31d..1cb97cf6 100644
--- a/chapter17/src/test/java/com/seaofnodes/simple/TypeTest.java
+++ b/chapter17/src/test/java/com/seaofnodes/simple/TypeTest.java
@@ -20,7 +20,7 @@ public class TypeTest {
                 Field.make("b", TypeInteger.BOT,-4, false) });
         Assert.assertEquals(s1, s1.glb());
         Assert.assertNotEquals(s1, s1.dual());
-        Assert.assertEquals(s1, s1.dual().glb());
+        Assert.assertEquals(s1.makeRO(), s1.dual().glb());
 
         TypeMem m1 = TypeMem.make(1,TypeInteger.ZERO);
         TypeMem m2 = TypeMem.make(2,TypeInteger.U16);
@@ -57,7 +57,7 @@ public class TypeTest {
         Assert.assertEquals(ptr1nil, ptr1.glb());
 
         Assert.assertEquals(ptr1, ptr1.dual().dual());
-        Assert.assertEquals(ptr1.glb(), ptr1.dual().glb());
+        Assert.assertEquals(ptr1.glb().makeRO(), ptr1.dual().glb());
         Assert.assertEquals(TypeMemPtr.make(TypeStruct.BOT,true), ptr1.meet(ptr2nil));
         Assert.assertEquals(ptr1.glb(), ptr1.meet(TypeMemPtr.NULLPTR));
 
diff --git a/chapter17/src/test/java/com/seaofnodes/simple/evaluator/Evaluator.java b/chapter17/src/test/java/com/seaofnodes/simple/evaluator/Evaluator.java
index 37d2a9b1..72f84fa2 100644
--- a/chapter17/src/test/java/com/seaofnodes/simple/evaluator/Evaluator.java
+++ b/chapter17/src/test/java/com/seaofnodes/simple/evaluator/Evaluator.java
@@ -308,6 +308,7 @@ public class Evaluator {
             case CProjNode    cproj -> ((Object[])val(cproj.ctrl()))[cproj._idx];
             case ProjNode     proj  -> ((Object[])val( proj.in(0) ))[ proj._idx];
             case ScopeMinNode mem   -> null;
+            case ReadOnlyNode ro    -> val(ro.in(1));
             default                 -> throw new AssertionError("Unexpected node " + node);
         };
     }