code: function (command) {
      if (!command || !_.isObject(command)) {
        var err = new Error('Please inject a valid command object!');
        debug(err);
        throw err;
      }
  
      if (!command.payload && command.payload !== '') {
        command.payload = this.defaultCommandPayload;
      }
  
      command.defineAggregate(this);
  
      if (this.commands.indexOf(command) < 0) {
        this.commands.push(command);
      }
    }
docs: Add command module.
  @param {Command} command The command module to be injected.