code: func Reduce(err error) error {
  	if agg, ok := err.(Aggregate); ok && err != nil {
  		switch len(agg.Errors()) {
  		case 1:
  			return agg.Errors()[0]
  		case 0:
  			return nil
  		}
  	}
  	return err
  }
docs: // Reduce will return err or, if err is an Aggregate and only has one item,
  // the first item in the aggregate.