Update README.md

1 files changed, 22 insertions, 2 deletions

diff --git a/chapter13/README.md b/chapter13/README.md
index 49737663..892f7170 100644
--- a/chapter13/README.md
+++ b/chapter13/README.md
@@ -57,10 +57,30 @@ struct Person {
   FamilyTree tree?;  // A family tree, or not
 }
 struct FamilyTree {
-  Person father?;
+  Person father?;    // FamilyTree can reference Person
   Person mother?;
   Person []kids?;
 }
 ```
 
-Normal field syntax works:  `person.tree.father = new Person;` or `person.tree.father.name = "Dad";`.
+Normal field syntax works:  
+e.g. `person.tree.father = new Person;` or 
+     `person.tree.father.name = "Dad";`.
+
+Reference fields all start out null, and can be tested as such.  However, a
+non-null reference field can only be stored with a non-null ref.
+
+```java
+struct N { N next; int i; }
+N n = new N;
+return n.next;  // Value reports as null, despite field typed as not-null
+```
+
+And:
+
+```java
+struct N { N next; int i; }
+N n = new N;
+n.next = null; // <<-- Compile error, cannot store null into not-null field
+return n.next;
+```