static <A, B, C extends Iterator<B>> ItIt<B> nestedIterator(final Collection<A> c, final F1<A, C> makeInnerIterator) {
  ret nestedIterator(iterator(c), makeInnerIterator);
}

static <A, B, C extends Iterator<B>> ItIt<B> nestedIterator(final Iterator<A> it1, final F1<A, C> makeInnerIterator) {
  if (it1 == null || !it1.hasNext()) ret emptyIterableIterator();
  ret iff(new F0 {
    A a;
    Iterator<B> innerIterator;
    
    { nextOuter(); }
    
    void nextOuter {
      a = it1.next();
      innerIterator = callF(makeInnerIterator, a);
    }
    
    O get() {
      while true { ping();
        if (innerIterator.hasNext())
          ret innerIterator.next();
        if (!it1.hasNext()) ret endMarker();
        nextOuter();
      }
    }
  });
}