code: function findFirstTabbableOfPreviousGroup($FirstTabbableInScope, $Tabbables, oSouceGroup, bFindPreviousGroup) {
  			var oGroup, $Target;
  
  			for (var i = $Tabbables.length - 1; i >= 0; i--) {
  				oGroup = findClosestGroup($Tabbables[i]);
  				if (oGroup != oSouceGroup) {
  					if (bFindPreviousGroup) {
  						//First find last tabbable of previous group and remember this new group (named "X" in the following comments)
  						oSouceGroup = oGroup;
  						bFindPreviousGroup = false;
  					} else {
  						//Then starting from group X and try to find again the last tabbable of previous group (named "Y")
  						//-> Jump one tabbable back to get the first tabbable of X
  						$Target = jQuery($Tabbables[i + 1]);
  						break;
  					}
  				}
  			}
  
  			if (!$Target && !bFindPreviousGroup) {
  				//Group X found but not group Y -> X is the first group -> Focus the first tabbable scope (e.g. page) element
  				$Target = $FirstTabbableInScope;
  			}
  
  			return $Target;
  		}
docs: see navigate() (bForward = false)