code: func (args Args) Validate(accepted map[string]bool) error {
  	for name := range args.fields {
  		if !accepted[name] {
  			return invalidFilter(name)
  		}
  	}
  	return nil
  }
docs: // Validate compared the set of accepted keys against the keys in the mapping.
  // An error is returned if any mapping keys are not in the accepted set.