docs: function(hierarchyName){
          if (_isUnd(hierarchyName)) return this._hierarchyIndex;
          var index = this._hierarchyIndexes[hierarchyName];
          if (_isUnd(index))
              Xmla.Exception._newError(
                  "INVALID_HIERARCHY",
                  "Xmla.Dataset.Axis.hierarchyDef",
                  hierarchyName
              )._throw();
          return index;
      }