static CharacterIterator characterIterator(S s, int from default 0) {
  ret new CharacterIterator {
    int n = l(s);
    int i = from;
    
    public bool hasNext() { ret i < n; }
    public char next() { ret s.charAt(i++); }
  };
}

static CharacterIterator characterIterator(S s, int from, int _n) {
  ret new CharacterIterator {
    int n = min(l(s), from+_n);
    int i = from;
    
    public bool hasNext() { ret i < n; }
    public char next() { ret s.charAt(i++); }
  };
}

static CharacterIterator characterIterator(char[] s, int from, int _n) {
  ret new CharacterIterator {
    int n = min(l(s), from+_n);
    int i = from;
    
    public bool hasNext() { ret i < n; }
    public char next() { ret s[i++]; }
  };
}