}

	// The "pre" output should all have been flushed already.
	if want, got := pre, out.String(); want != got {
		t.Errorf("filter should have already flushed %q, but flushed %q", want, got)
	}

	code, err := f.Finish()
	if err != nil {
		t.Fatal(err)
	}

	// Nothing more should have been written to out.
	if want, got := pre, out.String(); want != got {
		t.Errorf("want output %q, got %q", want, got)
	}