Passing results in prolog

sum([],0).

sum([N|L2],S) :- sum(L2,S2), plus(N,S2,S).

p([],1).

p([LL|L2],R) :- sum(LL,S), p(L2,P), mult(S,P,R).

prodsumlist([], 1).

prodsumlist([Head | Tail], Result) :-
    sumlist(Head, Sum_Of_Head),
    prodsumlist(Tail, ProdSum_Of_Tail),
    Result is Sum_Of_Head * ProdSum_Of_Tail.

?- prodsumlist([[1, 2], [3], [-4]], Result).
Result = -36.

prodsumlist(List, Result) :-
    xprodsumlist(List,1,Result).

xprodsumlist([],R,R).

xprodsumlist([Head|Rest],Sofar,Result) :-
    sumlist(Head, Sum_Of_Head),
    NewSofar is Sofar * Sum_Of_Head,
    xprodsumlist(Rest, NewSofar, Result).