docs: function(index){
          if (_isUnd(index)) index = this._hierarchyIndex;
          if (index !== parseInt(index, 10) ||
              index >= this.numHierarchies
          )
              Xmla.Exception._newError(
                  "INVALID_HIERARCHY",
                  "Xmla.Dataset.Axis.hierarchyDef",
                  index
              )._throw();
          return this._hierarchyOrder[index];
      }