static ItIt<Char> charIterator(S s, int from default 0) {
  ret countIterator_exclusive(from, l(s), i -> s.charAt(i));
}

static ItIt<Char> charIterator(S s, int from, int n) {
  ret countIterator_exclusive(from, min(l(s), from+n), i -> s.charAt(i));
}

static ItIt<Char> charIterator(char[] s, int from, int n) {
  ret countIterator_exclusive(from, min(l(s), from+n), i -> s[i]);
}