code: func (args Args) WalkValues(field string, op func(value string) error) error {
  	if _, ok := args.fields[field]; !ok {
  		return nil
  	}
  	for v := range args.fields[field] {
  		if err := op(v); err != nil {
  			return err
  		}
  	}
  	return nil
  }
docs: // WalkValues iterates over the list of values for a key in the mapping and calls
  // op() for each value. If op returns an error the iteration stops and the
  // error is returned.