union method
Returns a set containing all distinct elements from both collections.
The returned set preserves the element iteration order of the original collection.
Those elements of the other
collection that are unique are iterated in the end
in the order of the other
collection.
Implementation
KtSet<T> union(KtIterable<T> other) {
final set = toMutableSet();
set.addAll(other);
return set;
}