code: function (commandHandler) {
      if (!commandHandler || !_.isObject(commandHandler) || !_.isFunction(commandHandler.useAggregate)) {
        var err = new Error('Please inject a valid commandHandler object!');
        debug(err);
        throw err;
      }
  
      commandHandler.useAggregate(this);
  
      if (this.commandHandlers.indexOf(commandHandler) < 0) {
        this.commandHandlers.push(commandHandler);
      }
    }
docs: Add commandHandler module.
  @param {CommandHandler} commandHandler The commandHandler module to be injected.