https://lists.gnome.org/archives/gtkmm-list/2004-July/msg00003.html
Re: [gtkmm] Example code for TreeSortable::set_sort_func()
example codesetsortfunc
https://gnome.pages.gitlab.gnome.org/gtkmm/classGtk_1_1TreeSortable.html
gtkmm: Gtk::TreeSortable Class Reference
gtkclassreference