code: function(sPropertyName) {
  			var oResult = null;
  			for (var i = -1, oCurrentSorter; (oCurrentSorter = this._aSortCondition[++i]) !== undefined;) {
  				if (oCurrentSorter.property.name === sPropertyName) {
  					oResult = {
  						sorter : oCurrentSorter,
  						index : i
  					};
  					break;
  				}
  			}
  			return oResult;
  		}
docs: Checks if an order by expression for the given property is already
  defined and returns a reference to an object with property sorter and
  index of the object or null if the property is not yet defined in an
  order by expression.
  
  @private