function _fnSortListener ( settings, colIdx, append, callback )
	{
		var col = settings.aoColumns[ colIdx ];
		var sorting = settings.aaSorting;
		var asSorting = col.asSorting;
		var nextSortIdx;
		var next = function ( a, overflow ) {
			var idx = a._idx;
			if ( idx === undefined ) {
				idx = $.inArray( a[1], asSorting );
			}
	
			return idx+1 < asSorting.length ?
				idx+1 :
				overflow ?
					null :
					0;
		};
	
		// Convert to 2D array if needed
		if ( typeof sorting[0] === 'number' ) {
			sorting = settings.aaSorting = [ sorting ];
		}
	
		// If appending the sort then we are multi-column sorting
		if ( append && settings.oFeatures.bSortMulti ) {
			// Are we already doing some kind of sort on this column?
			var sortIdx = $.inArray( colIdx, _pluck(sorting, '0') );
	
			if ( sortIdx !== -1 ) {
				// Yes, modify the sort
				nextSortIdx = next( sorting[sortIdx], true );
	
				if ( nextSortIdx === null && sorting.length === 1 ) {
					nextSortIdx = 0; // can't remove sorting completely
				}
	
				if ( nextSortIdx === null ) {
					sorting.splice( sortIdx, 1 );
				}
				else {
					sorting[sortIdx][1] = asSorting[ nextSortIdx ];
					sorting[sortIdx]._idx = nextSortIdx;
				}
			}
			else {
				// No sort on this column yet
				sorting.push( [ colIdx, asSorting[0], 0 ] );
				sorting[sorting.length-1]._idx = 0;
			}
		}
		else if ( sorting.length && sorting[0][0] == colIdx ) {
			// Single column - already sorting on this column, modify the sort
			nextSortIdx = next( sorting[0] );
	
			sorting.length = 1;
			sorting[0][1] = asSorting[ nextSortIdx ];
			sorting[0]._idx = nextSortIdx;
		}
		else {
			// Single column - sort only on this column
			sorting.length = 0;
			sorting.push( [ colIdx, asSorting[0] ] );
			sorting[0]._idx = 0;
		}
	
		// Run the sort by calling a full redraw
		_fnReDraw( settings );
	
		// callback used for async user interaction
		if ( typeof callback == 'function' ) {
			callback( settings );
		}
	}
Function to run on user sort request
 @param {object} settings dataTables settings object
 @param {node} attachTo node to attach the handler to
 @param {int} colIdx column sorting index
 @param {boolean} [append=false] Append the requested sort to the existing
   sort if true (i.e. multi-column sort)
 @param {function} [callback] callback function
 @memberof DataTable#oApi