Prolog, building list with conditional clauses

Question

I need to do this homework assignment using prolog (SWI-flavor) and cant get my head around some things.

For example, if i want to iterate through a list and add it

Answer 1

try the ignore/1 predicate:

goal(Stuff) :-
    do_something(X)
    ignore(only_do_this_if_something(Y)),
    always_do_this(Z).

ignore/1 calls the only argument and succeeds whenever it fails or not:

ignore(X) :- X, !.
ignore(_).