// problematic types such as pointers. Note that it only does special handling
// for the troublesome types used in the test cases; it is not a general
// printer.
func sprintKey(key reflect.Value) string {
	switch str := key.Type().String(); str {
	case "*int":
		ptr := key.Interface().(*int)
		for i := range ints {
			if ptr == &ints[i] {
				return fmt.Sprintf("PTR%d", i)
			}
		}
		return "PTR???"
	case "unsafe.Pointer":