regexp.go
func Grep(r string) Filter {
	re, err := regexp.Compile(r)
	if err != nil {
		return FilterFunc(func(Arg) error { return err })
	}
	return If(re.MatchString)
}
func Grep(r string) Filter {
	re, err := regexp.Compile(r)
	if err != nil {
		return FilterFunc(func(Arg) error { return err })
	}
	return If(re.MatchString)
}
// Grep emits every input x that matches the regular expression r.