Flatten a list in Prolog

Question

I\'ve only been working with Prolog for a couple days. I understand some things but this is really confusing me.

I\'m suppose to write a function that takes a list

Answer 1

Without any other predicate, with tail-recursion only.

flatten([[X|S]|T], F) :- flatten([X|[S|T]], F).
flatten([[]|S], F) :- flatten(S, F).
flatten([X|S], [X|T]) :- \+(X = []), \+(X = [_|_]), flatten(S, T).
flatten([], []).