Now loads the theory "Let". Could add it to FOL, but this appears to
be incompatible with CCL.

--- a/src/ZF/ROOT.ML	Sat Apr 22 13:25:31 1995 +0200
+++ b/src/ZF/ROOT.ML	Tue Apr 25 10:56:49 1995 +0200
@@ -32,6 +32,7 @@
 use     "../Pure/section_utils.ML";
 use     "thy_syntax.ML";
 
+use_thy "Let";
 use_thy "InfDatatype";
 use_thy "List";
 use_thy "EquivClass";