static IterableIterator<S> it_observePrefix(final Iterator<S> src) {
  ret iteratorFF(new F0 {
    S prefix;
    
    O get() {
      if (!src.hasNext()) ret endMarker();
      S s = src.next();
      prefix = prefix == null ? s : commonPrefix(prefix, s);
      ret prefix;
    }
  });
}