Also don't shadow combinators.

(I realized that the way defs are parsed now means that each def
(but the last) is first asserted with the wrong definition expression
(it includes the symbol of the following definition at the end) and
then the parser figures out that there's another defintion following
and re-asserts the correct expression.  It would be nice to fix that
but it's kind of a PITA.  I used to build a list of definitions and
then assert them all at the end.  For now there aren't enough defs to
justify the extra work.

thun/thun.pl

@@ -213,6 +213,7 @@ assert_defs(DefsFile) :-
 
 assert_def(def(Def, Body)) :-
     \+ func(Def, _, _),
+    \+ combo(Def, _, _, _, _),
     retractall(def(Def, _)),
     assertz(def(Def, Body)).