// f: func -> A (stream ends when f returns null)
static <A> IterableIterator<A> iteratorFromFunction(fO f) {
  ret new IterableIterator<A>() {
    A a;
    bool done;
    
    public bool hasNext() {
      getNext();
      ret !done;
    }
    
    public A next() {
      getNext();
      if (done) fail();
      A _a = a;
      a = null;
      ret _a;
    }
    
    void getNext() {
      if (done || a != null) ret;
      a = (A) callF(f);
      done = a == null;
    }
  };
}