* - Proper coverage checking to see if the unit tests cover
* all possible cases.
* - Check if the iterator stamp is incremented at the correct times.
+ *
+ * For more thorough testing:
+ * - Test with randomized models.
+ * - Extensively test a filter model wrapping a sort model,
+ * or a sort model wrapping a filter model by:
+ * # Checking structure.
+ * # Checking for correct signals emissions.
+ * # Checking correct reference counting.
+ * # Tests should be done with the sort and filter model
+ * in various filtering and sorting states.
*/