Hi there.  I have an NSTableView that's bound to an NSArrayController.  Everything gets loaded into the table correctly, and I have sort keys and selectors set for the various columns (this also works).  What I'm trying to do is sort multiple columns when the user clicks one column header, but I can't find any way to do this.  I could brute-force a way by overriding tableView:mouseDownInHeaderOfTableColumn:, but that seems an inelegant way of doing it.
Is there a good way of doing multi-column sorting in an NSTableView using bindings?