diff -r d48d1716bb6d -r f5d0a66c8124 doc-src/Exercises/2001/a3/Trie3.thy
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/doc-src/Exercises/2001/a3/Trie3.thy	Thu Dec 05 17:12:07 2002 +0100
@@ -0,0 +1,19 @@
+(*<*)
+theory Trie3 = Main:
+(*>*)
+
+text {* Instead of association lists we can also use partial functions
+that map letters to subtrees. Partiality can be modelled with the help
+of type @{typ "'a option"}: if @{term f} is a function of type
+\mbox{@{typ "'a \<Rightarrow> 'b option"}}, set @{term "f a = Some b"}
+if @{term a} should be mapped to some @{term b} and set @{term "f a =
+None"} otherwise.  *}
+
+datatype ('a, 'v) trie = Trie  "'v option" "'a \<Rightarrow> ('a,'v) trie option";
+
+text{* Modify the definitions of @{term lookup} and @{term modify}
+accordingly and show the same correctness theorem as above. *}
+
+(*<*)
+end;
+(*>*)