log.Printf("%s %x: %v", l.prefix, p[0:n], err)
	} else {
		log.Printf("%s %x", l.prefix, p[0:n])
	}
	return
}

// NewReadLogger returns a reader that behaves like r except
// that it logs (using [log.Printf]) each read to standard error,
// printing the prefix and the hexadecimal data read.
func NewReadLogger(prefix string, r io.Reader) io.Reader {
	return &readLogger{prefix, r}

	}()

	lOut := new(strings.Builder)
	log.SetPrefix("lr: ")
	log.SetOutput(lOut)
	log.SetFlags(0)

	data := []byte("Hello, World!")
	p := make([]byte, len(data))
	lr := bytes.NewReader(data)
	rl := NewReadLogger("read:", lr)

	n, err := rl.Read(p)
	if err != nil {
		t.Fatalf("Unexpectedly failed to read: %v", err)
	}

	if g, w := p[:n], data; !bytes.Equal(g, w) {