docs: def unpack(d):
      """Unpack and return a framed binary message.
  
      """
      p = SBP._parser.parse(d)
      assert p.preamble == SBP_PREAMBLE, "Invalid preamble 0x%x." % p.preamble
      return SBP(p.msg_type, p.sender, p.length, p.payload, p.crc)