# Pastebin q7xQGbeY
  fun ref rstrip(s: String box = " \t\v\f\r\n") =>
    """
    Remove all trailing characters within the string that are in s. By default,
    trailing whitespace is removed.
    """
    if _size > 0 then
      let chars = Array[U32](s.size())
      var i = _size - 1
      var truncate_at = i

      for rune in s.runes() do
        chars.push(rune)
      end

      repeat
        try
          match utf32(i.isize())?
          | (0xFFFD, 1) =>
            None
          | (let c: U32, _) =>
            if not chars.contains(c) then
              break
            end
            truncate_at = i
          end
        else
          break
        end
      until (i = i - 1) == 0 end

      truncate(truncate_at)
    end