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