t.Errorf("shrinking: expected %q got %q", expect, m)
	}

	// 3. Rot13
	m = Map(rot13, []byte("a to zed"))
	expect = "n gb mrq"
	if string(m) != expect {
		t.Errorf("rot13: expected %q got %q", expect, m)
	}

	// 4. Rot13^2
	m = Map(rot13, Map(rot13, []byte("a to zed")))
	expect = "a to zed"
	if string(m) != expect {
		t.Errorf("rot13: expected %q got %q", expect, m)
	}

	// 5. Drop