Takes a predicate pred, a list xs, and a list ys, and returns a list xs' comprising each of the elements of xs which is equal to one or more elements of ys according to pred.

pred must be a binary function expecting an element from each list.

xs, ys, and xs' are treated as sets, semantically, so ordering should not be significant, but since xs' is ordered the implementation guarantees that its values are in the same order as they appear in xs. Duplicates are not removed, so xs' may contain duplicates if xs contains duplicates.