firstlast.go
func Last(n int) Filter {
	return FilterFunc(func(arg Arg) error {
		r := newRing(n)
		for s := range arg.In {
			r.pushBack(s)
		}
		for !r.empty() {
			arg.Out <- r.popFront()
		}
		return nil
	})
}
func Last(n int) Filter {
	return FilterFunc(func(arg Arg) error {
		r := newRing(n)
		for s := range arg.In {
			r.pushBack(s)
		}
		for !r.empty() {
			arg.Out <- r.popFront()
		}
		return nil
	})
}
// Last yields the last n items that it receives.