Applied fix for #12 to chapter 3 (#15)

ScopeNode added to chapter 3
Revised the description for Chapter 3 and 4 to include ScopeNode.

9 files changed, 287 insertions, 134 deletions

diff --git a/chapter03/README.md b/chapter03/README.md
index 31f3d46a..3e6cebfe 100644
--- a/chapter03/README.md
+++ b/chapter03/README.md
@@ -17,7 +17,7 @@ Here is the [complete language grammar](docs/03-grammar.md) for this chapter.
 
 ## Extensions to Intermediate Representation
 
-In this chapter we do not add new nodes. To recap, our list of Nodes are:
+To recap, our list of Nodes are:
 
 | Node Name  | Type    | Description                        | Inputs                                                           | Value                                                 |
 |------------|---------|------------------------------------|------------------------------------------------------------------|-------------------------------------------------------|
@@ -30,6 +30,13 @@ In this chapter we do not add new nodes. To recap, our list of Nodes are:
 | Div        | Data    | Divide a value by another          | Two data nodes, values are divided, order matters                | Result of the division                                |
 | UnaryMinus | Data    | Negate a value                     | One data node, value is negated                                  | Result of the unary minus                             |
 
+In this chapter we introduce:
+
+| Node Name | Type | Description                    | Inputs                          | Value |
+|-----------|------|--------------------------------|---------------------------------|-------|
+| Scope     | ?    | Represents scopes in the graph | All nodes that define variables | None  |
+
+
 ## Symbol Tables
 
 To support variable declarations and lexical scopes, we introduce symbol tables in the parser.
@@ -42,6 +49,19 @@ Declaring a name adds it to the current symbol table.
 If a name is assigned to, then its mapping in the most recent symbol table is updated.
 If a name is accessed, its mapping is looked up in the symbol tables. The lookup goes up the stack of symbol tables.
 
+## ScopeNode
+
+We wrap the stack of symbol tables in a `ScopeNode`. When a name binding is added to a symbol table,
+we also make the newly added node an input in the `ScopeNode`. This means that the `ScopeNode` is a user of
+all nodes that define new names. 
+
+From an implementation point of view, the symbol table maps names to offsets in the ScopeNode's inputs.
+
+When a scope is exited, we not only pop the symbol table representing the scope, but also remove the relevant input nodes.
+
+In the visualization, we show the ScopeNode in a separate box; the stack of symbol tables is shown inside
+the box. We show the edge from each variable in the symbol table to the defining Node in the graph.
+
 ## Demonstration
 
 We show the output from following code.
@@ -68,7 +88,7 @@ see the graph as its initially constructed).
 
 ![Graph1](./docs/03-graph1.svg)
 
-The diagram shows the symbol tables in the bottom cluster. 
+The diagram shows the ScopeNode and the symbol tables in the bottom cluster. 
 Each table is annotated by its scope level, and shows the variables defined in that scope.
 
 * When we exit the nested scope on line 8, the symbol table at level 1 is popped.
diff --git a/chapter03/docs/03-graph3.gv b/chapter03/docs/03-graph3.gv
index da30744b..717024e3 100644
--- a/chapter03/docs/03-graph3.gv
+++ b/chapter03/docs/03-graph3.gv
@@ -3,22 +3,26 @@ digraph chapter03 {
 	subgraph cluster_Nodes {
 		Start1 [ shape=box style=filled fillcolor=yellow label="Start" ];
 		Con_2 [ label="#1" ];
-		Con_19 [ label="#8" ];
 		Con_3 [ label="#2" ];
-		Return20 [ shape=box style=filled fillcolor=yellow label="Return" ];
+		Con_19 [ label="#8" ];
 		Con_4 [ label="#3" ];
+		Return20 [ shape=box style=filled fillcolor=yellow label="Return" ];
 		Con_5 [ label="#4" ];
 	}
 	node [shape=plaintext];
 	subgraph cluster_0 {
 		scope0 [label=<
 			<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
-			<TR><TD BGCOLOR="aqua">0</TD><TD PORT="scope0_y0">y0</TD><TD PORT="scope0_x0">x0</TD><TD PORT="scope0_y1">y1</TD><TD PORT="scope0_x1">x1</TD></TR>
+			<TR><TD BGCOLOR="cyan">0</TD><TD PORT="scope0_y0">y0</TD><TD PORT="scope0_x0">x0</TD><TD PORT="scope0_y1">y1</TD><TD PORT="scope0_x1">x1</TD></TR>
 			</TABLE>>];
 	}
+	Con_2 -> Start1 [style=dotted];
+	Con_3 -> Start1 [style=dotted];
 	Con_19 -> Start1 [style=dotted];
+	Con_4 -> Start1 [style=dotted];
 	Return20 -> Start1 [color=red];
 	Return20 -> Con_19;
+	Con_5 -> Start1 [style=dotted];
 	edge [style=dashed color=cornflowerblue];
 	scope0:scope0_y0 -> Con_3;
 	scope0:scope0_x0 -> Con_2;
diff --git a/chapter03/docs/03-graph3.svg b/chapter03/docs/03-graph3.svg
index 3900b93d..3b720323 100644
--- a/chapter03/docs/03-graph3.svg
+++ b/chapter03/docs/03-graph3.svg
@@ -4,117 +4,141 @@
 <!-- Generated by graphviz version 2.43.0 (0)
  -->
 <!-- Title: chapter03 Pages: 1 -->
-<svg width="394pt" height="292pt"
- viewBox="0.00 0.00 394.00 292.00" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
-<g id="graph0" class="graph" transform="scale(1 1) rotate(0) translate(4 288)">
+<svg width="554pt" height="220pt"
+ viewBox="0.00 0.00 554.00 220.00" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+<g id="graph0" class="graph" transform="scale(1 1) rotate(0) translate(4 216)">
 <title>chapter03</title>
-<polygon fill="white" stroke="transparent" points="-4,4 -4,-288 390,-288 390,4 -4,4"/>
+<polygon fill="white" stroke="transparent" points="-4,4 -4,-216 550,-216 550,4 -4,4"/>
 <g id="clust1" class="cluster">
 <title>cluster_Nodes</title>
-<polygon fill="none" stroke="black" points="8,-80 8,-276 378,-276 378,-80 8,-80"/>
+<polygon fill="none" stroke="black" points="8,-8 8,-204 386,-204 386,-8 8,-8"/>
 </g>
 <g id="clust2" class="cluster">
 <title>cluster_0</title>
-<polygon fill="none" stroke="black" points="115,-8 115,-60 259,-60 259,-8 115,-8"/>
+<polygon fill="none" stroke="black" points="394,-8 394,-60 538,-60 538,-8 394,-8"/>
 </g>
 <!-- Start1 -->
 <g id="node1" class="node">
 <title>Start1</title>
-<polygon fill="yellow" stroke="black" points="220,-268 166,-268 166,-232 220,-232 220,-268"/>
-<text text-anchor="middle" x="193" y="-246.3" font-family="Times,serif" font-size="14.00">Start</text>
+<polygon fill="yellow" stroke="black" points="198,-196 144,-196 144,-160 198,-160 198,-196"/>
+<text text-anchor="middle" x="171" y="-174.3" font-family="Times,serif" font-size="14.00">Start</text>
 </g>
 <!-- Con_2 -->
 <g id="node2" class="node">
 <title>Con_2</title>
-<ellipse fill="none" stroke="black" cx="343" cy="-106" rx="27" ry="18"/>
-<text text-anchor="middle" x="343" y="-102.3" font-family="Times,serif" font-size="14.00">#1</text>
+<ellipse fill="none" stroke="black" cx="207" cy="-106" rx="27" ry="18"/>
+<text text-anchor="middle" x="207" y="-102.3" font-family="Times,serif" font-size="14.00">#1</text>
 </g>
-<!-- Con_19 -->
+<!-- Con_2&#45;&gt;Start1 -->
+<g id="edge1" class="edge">
+<title>Con_2&#45;&gt;Start1</title>
+<path fill="none" stroke="black" stroke-dasharray="1,5" d="M198.65,-123.24C194.46,-131.39 189.27,-141.47 184.53,-150.69"/>
+<polygon fill="black" stroke="black" points="181.27,-149.38 179.81,-159.88 187.49,-152.58 181.27,-149.38"/>
+</g>
+<!-- Con_3 -->
 <g id="node3" class="node">
+<title>Con_3</title>
+<ellipse fill="none" stroke="black" cx="135" cy="-106" rx="27" ry="18"/>
+<text text-anchor="middle" x="135" y="-102.3" font-family="Times,serif" font-size="14.00">#2</text>
+</g>
+<!-- Con_3&#45;&gt;Start1 -->
+<g id="edge2" class="edge">
+<title>Con_3&#45;&gt;Start1</title>
+<path fill="none" stroke="black" stroke-dasharray="1,5" d="M143.35,-123.24C147.54,-131.39 152.73,-141.47 157.47,-150.69"/>
+<polygon fill="black" stroke="black" points="154.51,-152.58 162.19,-159.88 160.73,-149.38 154.51,-152.58"/>
+</g>
+<!-- Con_19 -->
+<g id="node4" class="node">
 <title>Con_19</title>
-<ellipse fill="none" stroke="black" cx="216" cy="-178" rx="27" ry="18"/>
-<text text-anchor="middle" x="216" y="-174.3" font-family="Times,serif" font-size="14.00">#8</text>
+<ellipse fill="none" stroke="black" cx="43" cy="-106" rx="27" ry="18"/>
+<text text-anchor="middle" x="43" y="-102.3" font-family="Times,serif" font-size="14.00">#8</text>
 </g>
 <!-- Con_19&#45;&gt;Start1 -->
-<g id="edge1" class="edge">
+<g id="edge3" class="edge">
 <title>Con_19&#45;&gt;Start1</title>
-<path fill="none" stroke="black" stroke-dasharray="1,5" d="M210.43,-195.95C207.84,-203.82 204.7,-213.38 201.8,-222.21"/>
-<polygon fill="black" stroke="black" points="198.47,-221.13 198.68,-231.72 205.12,-223.32 198.47,-221.13"/>
+<path fill="none" stroke="black" stroke-dasharray="1,5" d="M63.28,-118.09C82.45,-128.57 111.64,-144.54 134.8,-157.2"/>
+<polygon fill="black" stroke="black" points="133.21,-160.32 143.67,-162.05 136.57,-154.18 133.21,-160.32"/>
 </g>
-<!-- Con_3 -->
-<g id="node4" class="node">
-<title>Con_3</title>
-<ellipse fill="none" stroke="black" cx="271" cy="-106" rx="27" ry="18"/>
-<text text-anchor="middle" x="271" y="-102.3" font-family="Times,serif" font-size="14.00">#2</text>
+<!-- Con_4 -->
+<g id="node5" class="node">
+<title>Con_4</title>
+<ellipse fill="none" stroke="black" cx="351" cy="-106" rx="27" ry="18"/>
+<text text-anchor="middle" x="351" y="-102.3" font-family="Times,serif" font-size="14.00">#3</text>
+</g>
+<!-- Con_4&#45;&gt;Start1 -->
+<g id="edge4" class="edge">
+<title>Con_4&#45;&gt;Start1</title>
+<path fill="none" stroke="black" stroke-dasharray="1,5" d="M329.82,-117.38C324.99,-119.65 319.84,-121.97 315,-124 279.07,-139.06 237.19,-154.21 207.89,-164.43"/>
+<polygon fill="black" stroke="black" points="206.38,-161.25 198.08,-167.84 208.68,-167.87 206.38,-161.25"/>
 </g>
 <!-- Return20 -->
-<g id="node5" class="node">
+<g id="node6" class="node">
 <title>Return20</title>
-<polygon fill="yellow" stroke="black" points="226,-124 160,-124 160,-88 226,-88 226,-124"/>
-<text text-anchor="middle" x="193" y="-102.3" font-family="Times,serif" font-size="14.00">Return</text>
+<polygon fill="yellow" stroke="black" points="102,-52 36,-52 36,-16 102,-16 102,-52"/>
+<text text-anchor="middle" x="69" y="-30.3" font-family="Times,serif" font-size="14.00">Return</text>
 </g>
 <!-- Return20&#45;&gt;Start1 -->
-<g id="edge2" class="edge">
+<g id="edge5" class="edge">
 <title>Return20&#45;&gt;Start1</title>
-<path fill="none" stroke="red" d="M187.59,-124.47C184.7,-134.71 181.45,-147.96 180,-160 178.08,-175.88 178.08,-180.12 180,-196 181.02,-204.47 182.93,-213.53 184.98,-221.72"/>
-<polygon fill="red" stroke="red" points="181.64,-222.77 187.59,-231.53 188.4,-220.97 181.64,-222.77"/>
+<path fill="none" stroke="red" d="M71.6,-52.16C75.16,-71.21 83.04,-102.14 99,-124 108.52,-137.04 122.19,-148.23 135.06,-156.92"/>
+<polygon fill="red" stroke="red" points="133.48,-160.07 143.79,-162.53 137.27,-154.18 133.48,-160.07"/>
 </g>
 <!-- Return20&#45;&gt;Con_19 -->
-<g id="edge3" class="edge">
+<g id="edge6" class="edge">
 <title>Return20&#45;&gt;Con_19</title>
-<path fill="none" stroke="black" d="M198.69,-124.3C201.25,-132.1 204.33,-141.49 207.18,-150.17"/>
-<polygon fill="black" stroke="black" points="203.93,-151.49 210.38,-159.9 210.58,-149.3 203.93,-151.49"/>
-</g>
-<!-- Con_4 -->
-<g id="node6" class="node">
-<title>Con_4</title>
-<ellipse fill="none" stroke="black" cx="115" cy="-106" rx="27" ry="18"/>
-<text text-anchor="middle" x="115" y="-102.3" font-family="Times,serif" font-size="14.00">#3</text>
+<path fill="none" stroke="black" d="M62.57,-52.3C59.6,-60.3 56.01,-69.98 52.71,-78.85"/>
+<polygon fill="black" stroke="black" points="49.37,-77.78 49.17,-88.38 55.94,-80.22 49.37,-77.78"/>
 </g>
 <!-- Con_5 -->
 <g id="node7" class="node">
 <title>Con_5</title>
-<ellipse fill="none" stroke="black" cx="43" cy="-106" rx="27" ry="18"/>
-<text text-anchor="middle" x="43" y="-102.3" font-family="Times,serif" font-size="14.00">#4</text>
+<ellipse fill="none" stroke="black" cx="279" cy="-106" rx="27" ry="18"/>
+<text text-anchor="middle" x="279" y="-102.3" font-family="Times,serif" font-size="14.00">#4</text>
+</g>
+<!-- Con_5&#45;&gt;Start1 -->
+<g id="edge7" class="edge">
+<title>Con_5&#45;&gt;Start1</title>
+<path fill="none" stroke="black" stroke-dasharray="1,5" d="M260.19,-119.19C245.23,-128.89 223.89,-142.72 205.84,-154.42"/>
+<polygon fill="black" stroke="black" points="203.77,-151.59 197.28,-159.97 207.58,-157.46 203.77,-151.59"/>
 </g>
 <!-- scope0 -->
 <g id="node8" class="node">
 <title>scope0</title>
-<polygon fill="aqua" stroke="transparent" points="131,-23 131,-44 147,-44 147,-23 131,-23"/>
-<polygon fill="none" stroke="black" points="131,-23 131,-44 147,-44 147,-23 131,-23"/>
-<text text-anchor="start" x="134" y="-29.8" font-family="Times,serif" font-size="14.00">0</text>
-<polygon fill="none" stroke="black" points="147,-23 147,-44 171,-44 171,-23 147,-23"/>
-<text text-anchor="start" x="150" y="-29.8" font-family="Times,serif" font-size="14.00">y0</text>
-<polygon fill="none" stroke="black" points="171,-23 171,-44 195,-44 195,-23 171,-23"/>
-<text text-anchor="start" x="174" y="-29.8" font-family="Times,serif" font-size="14.00">x0</text>
-<polygon fill="none" stroke="black" points="195,-23 195,-44 219,-44 219,-23 195,-23"/>
-<text text-anchor="start" x="198" y="-29.8" font-family="Times,serif" font-size="14.00">y1</text>
-<polygon fill="none" stroke="black" points="219,-23 219,-44 243,-44 243,-23 219,-23"/>
-<text text-anchor="start" x="222" y="-29.8" font-family="Times,serif" font-size="14.00">x1</text>
+<polygon fill="cyan" stroke="transparent" points="410,-23 410,-44 426,-44 426,-23 410,-23"/>
+<polygon fill="none" stroke="black" points="410,-23 410,-44 426,-44 426,-23 410,-23"/>
+<text text-anchor="start" x="413" y="-29.8" font-family="Times,serif" font-size="14.00">0</text>
+<polygon fill="none" stroke="black" points="426,-23 426,-44 450,-44 450,-23 426,-23"/>
+<text text-anchor="start" x="429" y="-29.8" font-family="Times,serif" font-size="14.00">y0</text>
+<polygon fill="none" stroke="black" points="450,-23 450,-44 474,-44 474,-23 450,-23"/>
+<text text-anchor="start" x="453" y="-29.8" font-family="Times,serif" font-size="14.00">x0</text>
+<polygon fill="none" stroke="black" points="474,-23 474,-44 498,-44 498,-23 474,-23"/>
+<text text-anchor="start" x="477" y="-29.8" font-family="Times,serif" font-size="14.00">y1</text>
+<polygon fill="none" stroke="black" points="498,-23 498,-44 522,-44 522,-23 498,-23"/>
+<text text-anchor="start" x="501" y="-29.8" font-family="Times,serif" font-size="14.00">x1</text>
 </g>
 <!-- scope0&#45;&gt;Con_2 -->
-<g id="edge5" class="edge">
+<g id="edge9" class="edge">
 <title>scope0:scope0_x0&#45;&gt;Con_2</title>
-<path fill="none" stroke="cornflowerblue" stroke-dasharray="5,2" d="M183,-45C183,-102.26 255.22,-55.55 307,-80 310.61,-81.71 314.26,-83.76 317.77,-85.96"/>
-<polygon fill="cornflowerblue" stroke="cornflowerblue" points="315.88,-88.9 326.13,-91.56 319.78,-83.09 315.88,-88.9"/>
+<path fill="none" stroke="cornflowerblue" stroke-dasharray="5,2" d="M462,-45C462,-94.6 290.16,-72.65 243,-88 241.08,-88.63 239.12,-89.33 237.17,-90.09"/>
+<polygon fill="cornflowerblue" stroke="cornflowerblue" points="235.39,-87.04 227.58,-94.19 238.15,-93.48 235.39,-87.04"/>
 </g>
 <!-- scope0&#45;&gt;Con_3 -->
-<g id="edge4" class="edge">
+<g id="edge8" class="edge">
 <title>scope0:scope0_y0&#45;&gt;Con_3</title>
-<path fill="none" stroke="cornflowerblue" stroke-dasharray="5,2" d="M159,-45C159,-82.19 202.71,-61.55 235,-80 238.3,-81.89 241.68,-83.99 244.99,-86.15"/>
-<polygon fill="cornflowerblue" stroke="cornflowerblue" points="243.22,-89.18 253.46,-91.9 247.15,-83.39 243.22,-89.18"/>
+<path fill="none" stroke="cornflowerblue" stroke-dasharray="5,2" d="M438,-45C438,-67.35 411.84,-55.23 390,-60 294.13,-80.94 265.19,-60.49 171,-88 169.06,-88.57 167.09,-89.23 165.13,-89.95"/>
+<polygon fill="cornflowerblue" stroke="cornflowerblue" points="163.39,-86.88 155.5,-93.97 166.08,-93.34 163.39,-86.88"/>
 </g>
 <!-- scope0&#45;&gt;Con_4 -->
-<g id="edge7" class="edge">
+<g id="edge11" class="edge">
 <title>scope0:scope0_x1&#45;&gt;Con_4</title>
-<path fill="none" stroke="cornflowerblue" stroke-dasharray="5,2" d="M231,-45C231,-83.81 184.88,-61.06 151,-80 147.51,-81.95 143.94,-84.15 140.47,-86.42"/>
-<polygon fill="cornflowerblue" stroke="cornflowerblue" points="138.44,-83.57 132.14,-92.09 142.37,-89.35 138.44,-83.57"/>
+<path fill="none" stroke="cornflowerblue" stroke-dasharray="5,2" d="M510,-45C510,-71.13 433.9,-89.82 387,-98.86"/>
+<polygon fill="cornflowerblue" stroke="cornflowerblue" points="386.09,-95.47 376.9,-100.75 387.37,-102.35 386.09,-95.47"/>
 </g>
 <!-- scope0&#45;&gt;Con_5 -->
-<g id="edge6" class="edge">
+<g id="edge10" class="edge">
 <title>scope0:scope0_y1&#45;&gt;Con_5</title>
-<path fill="none" stroke="cornflowerblue" stroke-dasharray="5,2" d="M207,-45C207,-103.98 132.44,-55.04 79,-80 75.38,-81.69 71.73,-83.74 68.22,-85.93"/>
-<polygon fill="cornflowerblue" stroke="cornflowerblue" points="66.21,-83.06 59.85,-91.53 70.11,-88.87 66.21,-83.06"/>
+<path fill="none" stroke="cornflowerblue" stroke-dasharray="5,2" d="M486,-45C486,-123.37 388.78,-61.59 315,-88 313.15,-88.66 311.26,-89.39 309.37,-90.16"/>
+<polygon fill="cornflowerblue" stroke="cornflowerblue" points="307.8,-87.02 300.03,-94.23 310.6,-93.44 307.8,-87.02"/>
 </g>
 </g>
 </svg>
diff --git a/chapter03/src/main/java/com/seaofnodes/simple/GraphVisualizer.java b/chapter03/src/main/java/com/seaofnodes/simple/GraphVisualizer.java
index 5f3f52d0..dc5358f4 100644
--- a/chapter03/src/main/java/com/seaofnodes/simple/GraphVisualizer.java
+++ b/chapter03/src/main/java/com/seaofnodes/simple/GraphVisualizer.java
@@ -1,9 +1,6 @@
 package com.seaofnodes.simple;
 
-import com.seaofnodes.simple.node.ConstantNode;
-import com.seaofnodes.simple.node.Control;
-import com.seaofnodes.simple.node.Node;
-import com.seaofnodes.simple.node.StartNode;
+import com.seaofnodes.simple.node.*;
 
 import java.util.*;
 
@@ -33,13 +30,13 @@ public class GraphVisualizer {
         nodes(sb, all);
         
         // Now the scopes, in a cluster no edges
-        scopes(sb, parser._scopes);
+        scopes(sb, parser._scope);
 
         // Walk the Node edges
         nodeEdges(sb, all);
 
         // Walk the Scope edges
-        scopeEdges(sb, parser._scopes);
+        scopeEdges(sb, parser._scope);
         
         sb.append("}\n");
         return sb.toString();
@@ -61,10 +58,10 @@ public class GraphVisualizer {
         sb.append("\t}\n");     // End Node cluster
     }
 
-    private void scopes( StringBuilder sb, Stack<HashMap<String,Node>> scopes) {
+    private void scopes( StringBuilder sb, ScopeNode scopenode) {
         sb.append("\tnode [shape=plaintext];\n");
         int level=0;
-        for( HashMap<String,Node> scope : scopes ) {
+        for( HashMap<String,Integer> scope : scopenode._scopes ) {
             sb.append("\tsubgraph cluster_").append(level).append(" {\n"); // Magic "cluster_" in the subgraph name
             String scopeName = makeScopeName(level);
             sb.append("\t\t").append(scopeName).append(" [label=<\n");
@@ -102,13 +99,13 @@ public class GraphVisualizer {
     }
     
     // Walk the scope edges
-    private void scopeEdges( StringBuilder sb, Stack<HashMap<String,Node>> scopes) {
+    private void scopeEdges( StringBuilder sb, ScopeNode scopenode ) {
         sb.append("\tedge [style=dashed color=cornflowerblue];\n");
         int level=0;
-        for( HashMap<String,Node> scope : scopes ) {
+        for( HashMap<String,Integer> scope : scopenode._scopes ) {
             String scopeName = makeScopeName(level);
             for( String name : scope.keySet() )
-                sb.append("\t").append(scopeName).append(":").append(makePortName(scopeName, name)).append(" -> ").append(scope.get(name).uniqueName()).append(";\n");
+                sb.append("\t").append(scopeName).append(":").append(makePortName(scopeName, name)).append(" -> ").append(scopenode.in(scope.get(name)).uniqueName()).append(";\n");
             level++;
         }
     }
@@ -126,9 +123,9 @@ public class GraphVisualizer {
         for( Node n : start._outputs )
             walk(all, n);
         // Scan symbol tables
-        for( HashMap<String,Node> scope : parser._scopes )
-            for (Node n : scope.values())
-                walk(all, n);
+        for( HashMap<String,Integer> scope : parser._scope._scopes )
+            for (Integer i : scope.values())
+                walk(all, parser._scope.in(i));
         return all.values();
     }
 
diff --git a/chapter03/src/main/java/com/seaofnodes/simple/Parser.java b/chapter03/src/main/java/com/seaofnodes/simple/Parser.java
index 13b25e60..12e156ed 100644
--- a/chapter03/src/main/java/com/seaofnodes/simple/Parser.java
+++ b/chapter03/src/main/java/com/seaofnodes/simple/Parser.java
@@ -26,15 +26,14 @@ public class Parser {
     private final Lexer _lexer;
 
     /**
-     * Stack of lexical scopes, each scope is a symbol table
-     * that binds variable names to Nodes.
-     * The top of this stack represents current scope.
+     * Stack of lexical scopes, each scope is a symbol table that binds
+     * variable names to Nodes.  The top of this stack represents current scope.
      */
-    public Stack<HashMap<String, Node>> _scopes;
+    public ScopeNode _scope;
 
     public Parser(String source) {
         _lexer = new Lexer(source);
-        _scopes = new Stack<>();
+        _scope = new ScopeNode();
         Node.reset();
         START = new StartNode();
     }
@@ -43,35 +42,6 @@ public class Parser {
         return (ReturnNode) parseBlock();
     }
 
-    // Create a new name in current scope
-    private Node define(String name, Node n) {
-        // new name
-        _scopes.lastElement().put(name, n);
-        return n;
-    }
-
-    // If the name is present in any scope, then redefine
-    private Node update(String name, Node n) {
-        for (int i = _scopes.size() - 1; i >= 0; i--) {
-            HashMap<String, Node> scope = _scopes.get(i);
-            Node old = scope.get(name);
-            if (old != null) { // Found prior def
-                if (old.nOuts() == 0) old.kill(); // Delete old ref
-                scope.put(name, n); // Update existing ref
-                return n;
-            }
-        }
-        throw error("Undefined name '" + name + "'");
-    }
-
-    // Lookup a name in all scopes
-    private Node lookup(String name) {
-        for (int i = _scopes.size() - 1; i >= 0; i--) {
-            var n = _scopes.get(i).get(name);
-            if (n != null) return n;
-        }
-        throw error("Undefined name '" + name + "'");
-    }
 
     /**
      * Parses a block
@@ -82,14 +52,14 @@ public class Parser {
      */
     private Node parseBlock() {
         // Enter a new scope
-        _scopes.push(new HashMap<>());
+        _scope.push();
         Node n = null;
         while (!match("}") && !_lexer.isEOF()) {
             Node n0 = parseStatement();
             if (n0 != null) n = n0; // Allow null returns from eg showGraph
         }
         // Exit scope
-        _scopes.pop();
+        _scope.pop();
         return n;
     }
 
@@ -140,7 +110,9 @@ public class Parser {
         var name = requireId();
         require("=");
         var expr = require(parseExpression(), ";");
-        return update(name, expr);
+        if( _scope.update(name, expr)==null )
+            throw error("Undefined name '" + name + "'");
+        return expr;
     }
 
     /**
@@ -155,7 +127,9 @@ public class Parser {
         var name = requireId();
         require("=");
         var expr = require(parseExpression(), ";");
-        return define(name, expr);
+        if( _scope.define(name,expr) == null )
+            throw error("Redefining name '" + name + "'");
+        return expr;
     }
 
     /**
@@ -217,7 +191,11 @@ public class Parser {
     private Node parsePrimary() {
         if (_lexer.isNumber()) return parseIntegerLiteral();
         if (match("(")) return require(parseExpression(), ")");
-        return lookup(requireId());
+        String name = requireId();
+        Node id = _scope.lookup(name);
+        if( id==null )
+            throw error("Undefined name '" + name + "'");
+        return id;
     }
 
     /**
diff --git a/chapter03/src/main/java/com/seaofnodes/simple/node/Node.java b/chapter03/src/main/java/com/seaofnodes/simple/node/Node.java
index 5528f296..6b7d673a 100644
--- a/chapter03/src/main/java/com/seaofnodes/simple/node/Node.java
+++ b/chapter03/src/main/java/com/seaofnodes/simple/node/Node.java
@@ -152,27 +152,65 @@ public abstract class Node {
      * @param idx which def to set
      * @param new_def the new definition
      */
-    void set_def(int idx, Node new_def ) {
+    Node set_def(int idx, Node new_def ) {
         Node old_def = in(idx);
-        if( old_def != null ) { // If the old def exists, remove a use->def edge
-            ArrayList<Node> outs = old_def._outputs;
-            int lidx = outs.size()-1; // Last index
-            
-            // This 1-line hack compresses an element out of an ArrayList
-            // without having to copy the contents.  The last element is
-            // stuffed over the deleted element, and then the size is reduced.            
-            outs.set(outs.indexOf(this),outs.get(lidx));
-            outs.remove(lidx);  // Reduce ArrayList size without copying anything
-            if( lidx == 0 )     // If we removed the last use, the old def is now dead
+        if( old_def == new_def ) return this; // No change
+        if( old_def != null &&  // If the old def exists, remove a def->use edge
+            old_def.delUse(this) ) // If we removed the last use, the old def is now dead
                 old_def.kill(); // Kill old def
-        }
         // Set the new_def over the old (killed) edge
         _inputs.set(idx,new_def);
-        // If new def is not null, add the corresponding use->def edge
+        // If new def is not null, add the corresponding def->use edge
         if( new_def != null )
             new_def._outputs.add(this);
+        // Return self for easy flow-coding
+        return this;
+    }
+
+    /**
+     * Add a new def to an existing Node.  Keep the edges correct by
+     * adding the corresponding <em>def->use</em> edge.
+     *
+     * @param new_def the new definition, appended to the end of existing definitions
+     * @return new_def for flow coding
+     */
+    Node add_def(Node new_def) {
+        // Add use->def edge
+        _inputs.add(new_def);
+        // If new def is not null, add the corresponding def->use edge
+        if( new_def != null )
+            new_def._outputs.add(this);
+        return new_def;
+    }
+
+
+    // Breaks the edge invariants, used temporarily
+    private void addUse(Node n) { _outputs.add(n); }
+
+    // Remove node 'use' from 'def's (i.e. our) output list, by compressing the list in-place.
+    // Return true if the output list is empty afterward.
+    // Error is 'use' does not exist; ok for 'use' to be null.
+    private boolean delUse( Node use ) {
+        ArrayList<Node> outs = _outputs;
+        int lidx = outs.size()-1; // Last index
+        // This 1-line hack compresses an element out of an ArrayList
+        // without having to copy the contents.  The last element is
+        // stuffed over the deleted element, and then the size is reduced.
+        outs.set(outs.indexOf(use),outs.get(lidx));
+        outs.remove(lidx);  // Reduce ArrayList size without copying anything
+        return lidx==0;
+    }
+
+    // Shortcut for "popping" n nodes.  A "pop" is basically a
+    // set_def(last,null) followed by lowering the nIns() count.
+    void pop_n(int n) {
+        for( int i=0; i<n; i++ ) {
+            Node old_def = _inputs.remove(_inputs.size()-1);
+            if( old_def != null &&     // If it exists and
+                    old_def.delUse(this) ) // If we removed the last use, the old def is now dead
+                old_def.kill();        // Kill old def
+        }
     }
-    
   
     /**
      * Current computed type for this Node.  This value changes as the graph
diff --git a/chapter03/src/main/java/com/seaofnodes/simple/node/ScopeNode.java b/chapter03/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
new file mode 100644
index 00000000..c7d6edb9
--- /dev/null
+++ b/chapter03/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
@@ -0,0 +1,89 @@
+package com.seaofnodes.simple.node;
+
+import com.seaofnodes.simple.type.TypeBot;
+
+import java.util.HashMap;
+import java.util.Stack;
+
+/**
+ * The Scope node is a purely parser helper - it tracks names to nodes with a
+ * stack of scopes.
+ */
+public class ScopeNode extends Node {
+
+    /**
+     * Stack of lexical scopes, each scope is a symbol table
+     * that binds variable names to Nodes.
+     * The top of this stack represents current scope.
+     */
+    public final Stack<HashMap<String, Integer>> _scopes = new Stack<>();
+  
+    @Override
+    public String label() { return "Scope"; }
+  
+    @Override
+    StringBuilder _print(StringBuilder sb) {
+        for( HashMap<String,Integer> scope : _scopes ) {
+            sb.append("[");
+            boolean first=true;
+            for( String name : scope.keySet() ) {
+                if( !first ) sb.append(", ");
+                first=false;
+                sb.append(name).append(":");
+                Node n = in(scope.get(name));
+                if( n==null ) sb.append("null");
+                else n._print(sb);
+            }
+            sb.append("]");
+        }
+        return sb;
+    }
+
+    @Override
+    public TypeBot compute() { return TypeBot.BOTTOM; }
+
+    @Override
+    public Node idealize() { return null; }
+
+    // Add an empty lexical scope
+    public void push() {
+        _scopes.push(new HashMap<>());
+    }
+
+    // Remove the current lexical scope, killing all unused nodes.
+    public void pop() {
+        HashMap<String,Integer> scope = _scopes.pop();
+        pop_n(scope.size());
+    }
+
+    // Create a new name in the current scope
+    public Node define( String name, Node n ) {
+        HashMap<String,Integer> scope = _scopes.lastElement();
+        if( scope.put(name,nIns()) != null )
+            return null;        // Double define
+        return add_def(n);
+    }
+
+    // Lookup a name in all scopes
+    public Node lookup(String name) {
+        for (int i = _scopes.size() - 1; i >= 0; i--) {
+            var idx = _scopes.get(i).get(name);
+            if (idx != null) return in(idx);
+        }
+        return null;
+    }
+
+
+    // If the name is present in any scope, then redefine
+    public Node update(String name, Node n) {
+        for (int i = _scopes.size() - 1; i >= 0; i--) {
+            HashMap<String, Integer> scope = _scopes.get(i);
+            Integer idx = scope.get(name);
+            if (idx != null)           // Found prior def
+                return set_def(idx,n); // Update def in scope
+        }
+        return null;
+    }
+
+
+}
diff --git a/chapter04/README.md b/chapter04/README.md
index 7e8a8eea..45d054de 100644
--- a/chapter04/README.md
+++ b/chapter04/README.md
@@ -33,6 +33,7 @@ Below is our list of Nodes from [Chapter 3](../chapter03/README.md):
 | Mul        | Data    | Multiply two values                | Two data nodes, values are multiplied, order not important       | Result of the multiply       |
 | Div        | Data    | Divide a value by another          | Two data nodes, values are divided, order matters                | Result of the division       |
 | UnaryMinus | Data    | Negate a value                     | One data node, value is negated                                  | Result of the unary minus    |
+| Scope      | ?       | Represents scopes in the graph     | All nodes that define variables                                  | None                         |
 
 ## Projection Nodes
 
@@ -228,7 +229,7 @@ The peephole method does following:
 private Node deadCodeElim(Node m) {
     // If self is going dead and not being returned here (Nodes returned
     // from peephole commonly have no uses (yet)), then kill self.
-    if( m != this && isDead() ) {
+    if( m != this && isUnused() ) {
         // Killing self - and since self recursively kills self's inputs we
         // might end up killing 'm', which we are returning as a live Node.
         // So we add a bogus extra null output edge to stop kill().
diff --git a/chapter04/src/main/java/com/seaofnodes/simple/Parser.java b/chapter04/src/main/java/com/seaofnodes/simple/Parser.java
index a6872106..147515f6 100644
--- a/chapter04/src/main/java/com/seaofnodes/simple/Parser.java
+++ b/chapter04/src/main/java/com/seaofnodes/simple/Parser.java
@@ -45,6 +45,8 @@ public class Parser {
 
     String src() { return new String( _lexer._input ); }
 
+
+
     public ReturnNode parse() {
         _scope.push();
         try {