Don't assert defs twice.

Each definition is getting parsed with the name of the next one as part
of its body, then the next one fails to parse and the thing backtracks.
So each definition (but the last) gets asserted twice.

    def(--,[1,-,?])
    def(--,[1,-])
    def(?,[dup,bool,++])
    def(?,[dup,bool])
    def(++,[1,+,anamorphism])
    def(++,[1,+])
    def(anamorphism,[[pop,[]],swap,[dip,swons],genrec,app1])
    def(anamorphism,[[pop,[]],swap,[dip,swons],genrec])
    def(app1,[grba,infrst,app2])
    def(app1,[grba,infrst])
    def(app2,[[grba,swap,grba,swap],dip,[infrst],cons,ii,app3])
    def(app2,[[grba,swap,grba,swap],dip,[infrst],cons,ii])

...and so on.

thun/thun.pl

@@ -186,10 +186,10 @@ func(>=, [A, B|S], [T|S]) :- B #>= A #<==> R, r_truth(R, T).
 func(<=, [A, B|S], [T|S]) :- B #=< A #<==> R, r_truth(R, T).
 func(<>, [A, B|S], [T|S]) :- B #\= A #<==> R, r_truth(R, T).
 
-
 r_truth(0, false).
 r_truth(1, true).
 
+
 /*
 Combinators
 */
@@ -260,20 +260,22 @@ Definitions
 
 joy_def(def(Def, Body)) --> symbol(Def), blanks, "==", joy_parse(Body).
 
-joy_def --> joy_def(Def), {ignore(assert_def(Def))}.
-
-joy_defs --> blanks, joy_def, blanks, joy_defs.
-joy_defs --> [].
+joy_defs([Def|Rest]) --> blanks, joy_def(Def), blanks, joy_defs(Rest).
+joy_defs([]) --> [].
 
 assert_defs(DefsFile) :-
     read_file_to_codes(DefsFile, Codes, []),
-    phrase(joy_defs, Codes).
+    phrase(joy_defs(Defs), Codes),
+    maplist(assert_def, Defs).
 
 assert_def(def(Def, Body)) :-
-    \+ func(Def, _, _),
-    \+ combo(Def, _, _, _, _),
-    retractall(def(Def, _)),
-    assertz(def(Def, Body)).
+    (  % Don't let Def "shadow" functions or combinators.
+        \+ func(Def, _, _),
+        \+ combo(Def, _, _, _, _)
+    ) -> (
+        retractall(def(Def, _)),
+        assertz(def(Def, Body))
+    ) ; true.  % Otherwise it's okay.
 
 :- assert_defs("defs.txt").