code: func (a *AggregateResult) Group(result ...interface{}) {
  	for i := range result {
  		resultVal := reflect.ValueOf(result[i])
  		if resultVal.Kind() != reflect.Ptr {
  			panic("result argument must be an address")
  		}
  
  		if i >= len(a.group) {
  			panic(fmt.Sprintf("There is not %d elements in the grouping", i))
  		}
  
  		resultVal.Elem().Set(a.group[i])
  	}
  }
docs: // Group returns the field grouped by in the query