code: func NumberLines() Filter {
  	return FilterFunc(func(arg Arg) error {
  		line := 1
  		for s := range arg.In {
  			arg.Out <- fmt.Sprintf("%5d %s", line, s)
  			line++
  		}
  		return nil
  	})
  }
docs: // NumberLines prefixes its item with its index in the input sequence
  // (starting at 1) followed by a space.