a GSequenceIter
the GSequenceItercompare used to compare iterators in the sequence. It is called with two iterators pointing into seq. It should return 0 if the iterators are equal, a negative value if the first iterator comes before the second, and a positive value if the second iterator comes before the first.
user data passed to cmp_func
Like g_sequence_sort_changed(), but uses a GSequenceIterCompareFunc instead of a GCompareDataFunc as the compare function. Since 2.14