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

% citypops('A',L).
% L = [1583000,10102,87321,null,144000,203000,118000,238000,51102]
% citypops('A',.(H,T)).
% H = 1583000
% T = [10102,87321,null,144000,203000,118000,238000,51102]

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

% Test:  ?- sum(N,[1,2,3,4,5]).      yields 15

citypopsum(C,X) :- citypops(C,B), sum(X,B).

% citypopsum('A',X).
% X = 2434525