:- include(mondial).
citypops(C,B) :- bagof(Pop,N^P^Lo^La^El^city(N,C,P,Pop,La,Lo,El),B).

sum1(X,[H|T]) :- sum1(Y,T), H \= null, Y \= null, X is H + Y.
sum1(H,[H|T]) :- sum1(null,T), H \= null.
sum1(X,[null|T]) :- sum1(X,T).
sum1(null,[]).

sum2(X,.(H,T)) :- sum2(Y,T), H \= null, Y \= null, X is H + Y.
sum2(H,.(H,T)) :- sum2(null,T), H \= null.
sum2(X,.(null,T)) :- sum2(X,T).
sum2(null,[]).

citypopsum(C,X,Y) :- citypops(C,B), sum1(X,B), sum2(Y,B).