FIXED: CLP(B): weighted_maximum/3 with repeated variables

Many thanks to @jburse for reporting this issue:

    https://github.com/triska/clpz/issues/15

Example:

    ?- sat(~(Y*X)), sat(Z=:=Y), weighted_maximum([7,2,5],[X,Y,Z],W).
       W = 7, Y = 0, X = 1, Z = 0
    ;  W = 7, Y = 1, X = 0, Z = 1
    ;  false.

src/lib/clpb.pl

@@ -1276,13 +1276,18 @@ weighted_maximum(Ws, Vars, Max) :-
         maplist(var_with_index, Vars, IVs),
         pairs_keys_values(Pairs0, IVs, Ws),
         keysort(Pairs0, Pairs1),
-        pairs_keys_values(Pairs1, IVs1, WeightsIndexOrder),
+        % sum linear combinations of repeated variables
+        group_pairs_by_key(Pairs1, Groups),
+        maplist(group_sumweights_pair, Groups, Pairs2),
+        pairs_keys_values(Pairs2, IVs1, WeightsIndexOrder),
         pairs_values(IVs1, VarsIndexOrder),
         % Pairs is a list of Var-Weight terms, in index order of Vars
         pairs_keys_values(Pairs, VarsIndexOrder, WeightsIndexOrder),
         bdd_maximum(BDD, Pairs, Max),
         max_labeling(BDD, Pairs).
 
+group_sumweights_pair((I-V)-Ws, (I-V)-W) :- sum_list(Ws, W).
+
 max_labeling(1, Pairs) :- max_upto(Pairs, _, _).
 max_labeling(node(_,Var,Low,High,Aux), Pairs0) :-
         max_upto(Pairs0, Var, Pairs),