// iterators are not thread-safe
static <A> L<IterableIterator<A>> splitIterator(final int n, final Iterator<A> it) {
  class SplitIterator {
    final int[] positions = repIntAsArray(n, -1);
    new CircularArrayList<A> buffer;
    int first;
    bool done;
  
    L<IterableIterator<A>> iterators() {
      new L<IterableIterator<A>> l;
      for _i to n: {
        final int i = _i;
        l.add((IterableIterator) iteratorFF(func {
          positions[i]++;
          updateBuffer();
          int idx = positions[i]-first;
          ret idx >= l(buffer) ? endMarker() : buffer.get(idx);
        }));
      }
      ret l;
    }
    
    int last() { ret first+l(buffer); }
    
    void updateBuffer {
      int min = min(positions), max = max(positions);
      while (min > first) { buffer.remove(0); ++first; }
      while (!done && max >= last()) {
        if (!it.hasNext())
          done = true;
        else
          buffer.add(it.next());
      }
    }
  }
  ret new SplitIterator().iterators();
}