code: function (aggregate, cmd) {
      if (!aggregate || !_.isObject(aggregate)) {
        var err = new Error('Please pass a valid aggregate!');
        debug(err);
        throw err;
      }
      if (!cmd || !_.isObject(cmd)) {
        var err = new Error('Please pass a valid command!');
        debug(err);
        throw err;
      }
  
      var reason = this.isAggregateDestroyed(aggregate, cmd);
      if (reason) {
        return reason;
      }
  
      if (this.aggregate.disablePersistence) return;
  
      reason = this.isRevisionWrong(aggregate, cmd);
      if (reason) {
        return reason;
      }
    }
docs: Returns an error if verification fails.
  @param {AggregateModel} aggregate The passed aggregate.
  @param {Object}         cmd       The command.
  @returns {Error}