code: function(rule){
      rule = basis.getter(rule || UNION);
  
      if (this.rule !== rule)
      {
        var oldRule = this.rule;
  
        this.rule = rule;
        this.emit_ruleChanged(oldRule);
  
        return this.applyRule();
      }
    }
docs: Set new merge rule for dataset. Some types are available in basis.data.Dataset.Merge
  @param {function(count:number, sourceCount:number):boolean|string} rule New rule.
  @return {Object} Delta of member changes.