regexp.go
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) })
}
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) })
}
// GrepNot emits every input x that does not match the regular expression r.