static int lCommonPrefixOfCharIterators(CharacterIterator it1, CharacterIterator it2) {
  int n = 0;
  while true {
    if (!it1.hasNext() || !it2.hasNext()) ret n;
    char a = it1.next(), b = it2.next();
    if (a != b) ret n;
    ++n;
  }
}

static int lCommonPrefixOfCharIterators(CharacterIterator it1, CharacterIterator it2, CharComparator comparator) {
  if (comparator == null) ret lCommonPrefixOfCharIterators(it1, it2);
  int n = 0;
  while true {
    if (!it1.hasNext() || !it2.hasNext()) ret n;
    char a = it1.next(), b = it2.next();
    if (comparator.compare(a, b) != 0) ret n;
    ++n;
  }
}