delay attributed variables goal, naively correct dif/2

src/prolog/lib/dif.pl

@@ -1,6 +1,7 @@
 :- module(dif, [dif/2]).
 
 :- use_module(library(atts)).
+:- use_module(library(lists), [append/3]).
 
 :- attribute dif/1.
 
@@ -16,8 +17,20 @@ 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)) -> true
+    (   get_atts(Var, +dif(Goals)) ->
+	term_variables(Value, ValueVars),
+	append_goals(ValueVars, Goals)
     ;   Goals = []
     ).
 

src/prolog/lib/lists.pl

@@ -13,7 +13,7 @@ length(Xs, N) :-
     N >= 0, !,
     '$skip_max_list'(M, N, Xs, Xs0),
     (  Xs0 == [] -> N = M
-    ;  var(Xs0)  -> R is N-M, length_rundown(Xs0, R)).
+    ;  var(Xs0)  -> R is N-M, nl, length_rundown(Xs0, R)).
 length(_, N) :-
     integer(N), !,
     throw(error(domain_error(not_less_than_zero, N), length/2)).
@@ -25,7 +25,7 @@ length_addendum([_|Xs], N, M) :-
     M1 is M + 1,
     length_addendum(Xs, N, M1).
 
-length_rundown([], 0) :- !.
+length_rundown(Xs, 0) :- !, Xs = [].
 length_rundown([_|Xs], N) :-
     N1 is N-1,
     length_rundown(Xs, N1).