code: 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
  	})
  }
docs: // Last yields the last n items that it receives.