code: func WriteLines(writer io.Writer) Filter {
  	return FilterFunc(func(arg Arg) error {
  		for s := range arg.In {
  			if _, err := writer.Write(append([]byte(s),'\n')); err != nil {
  				return err
  			}
  			arg.Out <- s
  		}
  		return nil
  	})
  }
docs: // WriteLines prints each input item s followed by a newline to
  // writer; and in addition it emits s.  Therefore WriteLines()
  // can be used like the "tee" command, which can often be useful
  // for debugging.