Application: membership check in a difference list, to check whether we have reached the "end of the list"
[_,_,_,...,_|Back]-Back
from https://stackoverflow.com/questions/61086636/perfoming-member-check-on-a-difference-list-but-how
seen(Element, List-Back) :- List \== Back, List = [Element|_]. seen(Element, List-Back) :- List \== Back, List = [_| Tail], seen(Element, Tail-Back).