remove vestigial prolog/ directory (#444)

src/lib/dif.pl

@@ -0,0 +1,55 @@
+:- module(dif, [dif/2]).
+
+:- use_module(library(atts)).
+:- use_module(library(dcgs)).
+:- use_module(library(lists), [append/3]).
+
+:- attribute dif/1.
+
+put_dif_att(Var, X, Y) :-
+    (   get_atts(Var, +dif(Z)) ->
+	sort([X \== Y | Z], NewZ),
+	put_atts(Var, +dif(NewZ))
+    ;   put_atts(Var, +dif([X \== Y]))
+    ).
+
+dif_set_variables([], _, _).
+dif_set_variables([Var|Vars], X, Y) :-
+    put_dif_att(Var, X, Y),
+    dif_set_variables(Vars, X, Y).
+
+append_goals([], _).
+append_goals([Var|Vars], Goals) :-
+    (   get_atts(Var, +dif(VarGoals)) ->
+	append(Goals, VarGoals, NewGoals0),
+	sort(NewGoals0, NewGoals)
+    ;   NewGoals = Goals
+    ),
+    put_atts(Var, +dif(NewGoals)),
+    append_goals(Vars, Goals).
+
+verify_attributes(Var, Value, Goals) :-
+    (   get_atts(Var, +dif(Goals)) ->
+	term_variables(Value, ValueVars),
+	append_goals(ValueVars, Goals)
+    ;   Goals = []
+    ).
+
+% Probably the world's worst dif/2 implementation. I'm open to
+% suggestions for improvement.
+
+dif(X, Y) :- X \== Y,
+             (   term_variables(X, XVars), term_variables(Y, YVars),
+	         dif_set_variables(XVars, X, Y),
+		 dif_set_variables(YVars, X, Y)
+	     ).
+
+gather_dif_goals([]) --> [].
+gather_dif_goals([(X \== Y) | Goals]) -->
+    [dif(X, Y)],
+    gather_dif_goals(Goals).
+
+attribute_goals(X) -->
+    { get_atts(X, +dif(Goals)) },
+    gather_dif_goals(Goals),
+    { put_atts(X, -dif(_)) }.