code: def register_command(self, name, handler, validator):
          
  
          self._commands[name] = (handler, validator)
docs: Register a coroutine command handler.
  
          This handler will be called whenever a command message is received
          from the client, whose operation key matches ``name``.  The handler
          will be called as::
  
              response_payload = await handler(cmd_payload, context)
  
          If the coroutine returns, it will be assumed to have completed
          correctly and its return value will be sent as the result of the
          command.  If the coroutine wishes to signal an error handling the
          command, it must raise a ServerCommandError exception that contains a
          string reason code for the error.  This will generate an error
          response to the command.
  
          The cmd_payload is first verified using the SchemaVerifier passed in
          ``validator`` and handler is only called if verification succeeds. If
          verification fails, a failure response to the command is returned
          automatically to the client.
  
          Args:
              name (str): The unique command name that will be used to dispatch
                  client command messages to this handler.
              handler (coroutine function): A coroutine function that will be
                  called whenever this command is received.
              validator (SchemaVerifier): A validator object for checking the
                  command payload before calling this handler.