select(Item, Set1, Set2) and Set1, Set2 are both sorted lists
without duplicates. This implementation is only expected to work
for Item ground and either Set1 or Set2 ground. The "chk" suffix
is meant to remind you of memberchk/2, which also expects its
first argument to be ground. ord_selectchk(X, S, T) =>
ord_memberchk(X, S) & \+ ord_memberchk(X, T).