sclass Substring implements /*Comparable<? extends CharSequence>,*/ CharSequence {

  CharSequence s; // full string we reference

  int i, l; // starting index, length

  *() {}

  *(CharSequence *s, int *i, int j) { l = j-i; }

  *(CharSequence *s, int *i) { l = s.length()-i; }

  public bool equals(O o) {

    if (o cast CharSequence) {

      CharSequence c = o;

      if (l != c.length()) false;

      for idx to l:

        if (s.charAt(i+idx) != c.charAt(idx))

          false;

      true;

    }

    false;

  }

  toString {

    ret charSequenceToStringByChar(this);

  }

  public char charAt(int index) {

    if (index < 0 || index >= l) fail("Index out of range: " + index + " (length: " + l + ")");

    ret s.charAt(i+index);

  }

  public int length() { ret l; }

  public int startIndex() { ret i; }

  public int endIndex() { ret i+l; }

  public CharSequence baseString() { ret s; }

  IntRange range() { ret IntRange(startIndex(), endIndex()); }

  public CharSequence subSequence(int start, int end) {

    ret substring(start, end);

  }

  // TODO: index checking

  Substring substring(int start, int end) {

    ret Substring(s, i+start, i+end);

  }

  Substring substring(int start) {

    ret substring(start, l);

  }

  bool endsAtEnd() { ret i+l == s.length(); }

}