Define a Prolog predicate flatten(List, FlattenedList) that asserts List is any nested list of atoms and FlattenedList is the same list with the nesting removed. The atom [] should also be removed. Your predicate should only produce one answer. You may use the built-in predicates not, ! and append. Do not use a helper predicate.

?- flatten([a, [[b,c],d], [[e]], [f]], X).

X = [a,b,c,d,e,f] ;

no

?- flatten([a,[[]], [[c,d],e]], X).

X = [a,c,d,e] ;

no