}
		return nil
	}); err != nil {
		t.Fatal("comparing two directories:", err)
	}

	// Test with memory filesystem.
	fsys = fstest.MapFS{
		"william":    {Data: []byte("Shakespeare\n")},
		"carl":       {Data: []byte("Gauss\n")},
		"daVinci":    {Data: []byte("Leonardo\n")},
		"einstein":   {Data: []byte("Albert\n")},
		"dir/newton": {Data: []byte("Sir Isaac\n")},
	}