parameters: (self, command, handler)
docstring: Register a command handler; this handler will be called when a command
          is received.
  
          :param command: Command (string, case sensitive)
          :param handler: Handler function which will be called with (sequence number, payload)
          :return: None
docstring_summary: Register a command handler; this handler will be called when a command
          is received.
docstring_tokens: Register a command handler ; this handler will be called when a command is received .
function: def add_handler(self, command, handler):
          """
          Register a command handler; this handler will be called when a command
          is received.
  
          :param command: Command (string, case sensitive)
          :param handler: Handler function which will be called with (sequence number, payload)
          :return: None
          """
          self.handlers[command] = handler
function_tokens: def add_handler ( self , command , handler ) : self . handlers [ command ] = handler