correct attribute_goals/2 bug, add freeze/2, update README

src/prolog/lib/freeze.pl

@@ -0,0 +1,33 @@
+:- module(freeze, [freeze/2]).
+
+:- use_module(library(atts)).
+
+:- attribute frozen/1.
+
+verify_attributes(Var, Other, Goals) :-
+        get_atts(Var, frozen(Fa)), !,       % are we involved?
+        (   var(Other) ->                   % must be attributed then
+            (   get_atts(Other,  frozen(Fb)) % has a pending goal?
+            ->  put_atts(Other, -frozen(Fb)),
+	        put_atts(Other,  frozen((Fb,Fa))) % rescue conjunction
+            ;   put_atts(Other,  frozen(Fa)) % rescue the pending goal
+            ),
+            Goals = []
+        ;   Goals = [Fa]
+        ).
+verify_attributes(_, _, []).
+
+freeze(X, Goal) :-
+    put_atts(Fresh, frozen(Goal)),
+    Fresh = X.
+
+gather_freeze_goals(Attrs, _) :-
+    var(Attrs), !.
+gather_freeze_goals([frozen(X) | Attrs], [frozen(X) | Goals]) :-
+    gather_freeze_goals(Attrs, Goals).
+gather_freeze_goals([_ | Attrs], Goals) :-
+    gather_freeze_goals(Attrs, Goals).
+
+attribute_goals(X, Goals) :-
+    '$get_attr_list'(X, Attrs),
+    gather_freeze_goals(Attrs, Goals).