iter_cmp 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.
If you are simply searching for an existing element of the sequence, consider using Sequence.lookupIter.
This function will fail if the data contained in the sequence is unsorted.
data for the new item
the function used to compare iterators in the sequence
user data passed to iter_cmp
a glib.SequenceIter pointing to the position in seq where data would have been inserted according to iter_cmp and cmp_data