herbstluftwm : have super-tab cycle windows or go to last tag/workspace

If there is more than one window in current frame, cycle them. Else, go to previous workspace aka tag :

super=Mod4
hc keybind $super-Tab                                           \
            or , and . compare tags.focus.curframe_wcount ge 2  \
                     . cycle +1                                 \
               , use_previous