// f: func -> A (stream ends when f returns null)
static IterableIterator iff_null(fO f) {
ret iteratorFromFunction(f);
}
static IterableIterator iff_null(F0 f) {
ret iteratorFromFunction(f);
}
static IterableIterator iff_null(IF0 f) {
ret iteratorFromFunction(f);
}