Sequence.sortChanged

Moves the data pointed to by iter to a new position as indicated by cmp_func. This function should be called for items in a sequence already sorted according to cmp_func whenever some aspect of an item changes so that cmp_func may return different values for that item.

cmp_func is called with two items of the seq, and cmp_data. It should return 0 if the items are equal, a negative value if the first item comes before the second, and a positive value if the second item comes before the first.