code: func GrepNot(r string) Filter {
  	re, err := regexp.Compile(r)
  	if err != nil {
  		return FilterFunc(func(Arg) error { return err })
  	}
  	return If(func(s string) bool { return !re.MatchString(s) })
  }
docs: // GrepNot emits every input x that does not match the regular expression r.