if !strings.Contains(fmt.Sprint(err), wantsub) || n != 0 {
		t.Errorf("ReadAt(-10) = %v, %v; want 0, ...%q...", n, err, wantsub)
	}
}

func TestWriteAt(t *testing.T) {
	t.Parallel()

	f := newFile("TestWriteAt", t)
	defer Remove(f.Name())
	defer f.Close()

	const data = "hello, world\n"
	io.WriteString(f, data)

	n, err := f.WriteAt([]byte("WORLD"), 7)