How can I refresh sorting rules?

[Jarek Dukat <madmaxer poczta fm>]

I have a GtkTreeView. I call gtk_tree_sortable_set_default_sort_func()
with some function and then
gtk_tree_sortable_set_sort_column_id(GTK_DEFAULT_SORT_COLUMN_ID).
Sorting works well, but some time later I want to change sorting
function to some other one. But calling ...set_default_sort_func again
doesn't force the model to re-sort its data. How can I force it to do
so?

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