switch {
		case r >= 'A' && r <= 'Z':
			return 'A' + (r-'A'+13)%26
		case r >= 'a' && r <= 'z':
			return 'a' + (r-'a'+13)%26
		}
		return r
	}
	fmt.Printf("%s\n", bytes.Map(rot13, []byte("'Twas brillig and the slithy gopher...")))
	// Output:
	// 'Gjnf oevyyvt naq gur fyvgul tbcure...
}

func ExampleReader_Len() {
	fmt.Println(bytes.NewReader([]byte("Hi!")).Len())