docs: def calc_padding(fmt, align):
      """Calculate how many padding bytes needed for ``fmt`` to be aligned to
      ``align``.
  
      Args:
          fmt (str): :mod:`struct` format.
          align (int): alignment (2, 4, 8, etc.)
  
      Returns:
          str: padding format (e.g., various number of 'x').
  
      >>> calc_padding('b', 2)
      'x'
  
      >>> calc_padding('b', 3)
      'xx'
      """
      remain = struct.calcsize(fmt) % align
      if remain == 0:
          return ""
      return 'x' * (align - remain)