The first matching element is returned, or NULL if none found.
*/
-bool list_del(List *list, void *key, Comparator cmp, Callback call, void *arg);
+bool list_del(List *list, void *key, Comparator cmp, Callback call, void *arg, Transformer trans);
/*
Delete an element from the list if it is found.
Return whether an element has been deleted.