summands_sum([], 0).
summands_sum([I|Is], S1) :-
	sum(I, S2, S1),
	summands_sum(Is, S2).

summands_n_sum(Is,N,S) :-
	length(Is,N),
	summands_sum(Is, S).

:- pe summands_n_sum(Is, 6, S).
--**- 99.Bsp. n599 ------------------------
By adding the prefix operator pe we state that a goal should be evaluated partially.
**NEXT:[DO].
[logprog|GUPU|UWN]