remove vestigial prolog/ directory (#444)

src/lib/terms.pl

@@ -0,0 +1,19 @@
+:- module(terms, [numbervars/3]).
+
+:- use_module(library(error)).
+
+numbervars(Term, N0, N) :-
+   catch(internal_numbervars(Term, N0, N),
+	 error(E,Ctx),
+	 ( ( var(Ctx) -> Ctx = numbervars/3 ; true ), throw(error(E,Ctx) ) ) ).
+
+internal_numbervars(Term, N0, N) :-
+   must_be(integer, N0),
+   can_be(integer, N),
+   term_variables(Term, Vars),
+   numberlist(Vars, N0, N).
+
+numberlist([], N, N).
+numberlist(['$VAR'(N0)|Vars], N0, N) :-
+   N1 is N0+1,
+   numberlist(Vars, N1, N).