diff --git a/Thun.md b/Thun.md
index 56237da..b8d6d1f 100644
--- a/Thun.md
+++ b/Thun.md
@@ -11,6 +11,8 @@ programming language created by Manfred von Thun that is easy to use and
 understand and has many other nice properties.  **Thun** is a dialect of
 Joy that attempts to stay very close to the spirit of Joy but does not
 precisely match the behaviour of the original version written in C.
+(In the rest of this document I'll use the names Joy and Thun
+interchangably.)
 
 
 ## Grammar
@@ -150,11 +152,30 @@ of these:
 
 ### Definitions
 
-Thun can be extended by adding new definitions to the `defs.txt` file and
-rebuilding the binaries.  Each line in the file is a definition
+Thun can be extended by adding new definitions to the
+[defs.txt](https://git.sr.ht/~sforman/Thun/tree/trunk/item/implementations/defs.txt)
+file and rebuilding the binaries.  Each line in the file is a definition
 consisting of the new symbol name followed by an expression for the body
 of the function.
 
+The `defs.txt` file is just joy expression, one per line, that have a
+symbol followed by the definition for that symbol, e.g.:
+
+    sqr dup mul
+
+The definitions form a DAG (Directed Acyclic Graph) (there is actually a
+cycle in the definition of `genrec` but that's the point, it is a cycle
+to itself that captures the cyclical nature of recursive definitions.)
+
+I don't imagine that people will read `defs.txt` to understand Joy code.
+Instead people should read the notebooks that derive the functions to
+understand them.  The reference docs should help, and to that end I'd
+like to cross-link them with the notebooks.  The idea is that the docs
+are the code and the code is just a way to make precise the ideas in the
+docs.
+
+### Adding Functions to the Dictionary with `inscribe`
+
 You can use the `inscribe` command to put new definitions into the
 dictionary at runtime, but they will not persist after the program ends.
 The `inscribe` function is the only function that changes the dictionary.
@@ -162,6 +183,11 @@ It's meant for prototyping.  (You could abuse it to make variables by
 storing "functions" in the dictionary that just contain literal values as
 their bodies.)
 
+    [foo bar baz] inscribe
+
+This will put a definition for `foo` into the dictionary as `bar baz`.
+
+
 ## Problems
 
 ### Symbols as Data