build(L,S) :- build(L,[],S).

%%build([],A,S) :- S=A.

build([],A,A).
build([H|T],A,S) :- A2=n(H,A),build(T,A2,S).

build2(L,S) :- build2(L,S,X,X).

build2([],A,A,[]).
build2([H|T],S,A,X) :- X=n(H,X2),build2(T,S,A,X2).