docs: def nbytes(self):
          """The number of bytes required to encode this command.
  
          Encoded commands are comprised of a two byte opcode, followed by a
          one byte size, and then the command argument bytes.  The size
          indicates the number of bytes required to represent command
          arguments.
          """
          return len(self.opcode) + 1 + sum(arg.nbytes for arg in self.argdefns)