one of the documented procedures in this installation of the ACS
ad_reverse   order
What it does:
returns the opposite sort order from the one it is given. Mostly for columns whose natural sort order is not the default.
Defined in: /web/philip/packages/acs-core/table-display-procs.tcl

Source code:

    switch [string tolower $order] {
        desc {return asc}
        asc {return desc}
    return $order