code: def lower(self):
          
          # TODO : Bug in nested map operating on strings
          # TODO : Check that self.weld_type is a string type
          vectype = self.weld_type
          if isinstance(vectype, WeldVec):
              elem_type = vectype.elemType
              if isinstance(elem_type, WeldChar):
                          return SeriesWeld(
                              grizzly_impl.to_lower(
                                  self.expr,
                                  elem_type
                              ),
                              self.weld_type,
                              self.df,
                              self.column_name
                          )
          raise Exception("Cannot call to_lower on non string type")
docs: Summary
  
          Returns:
              TYPE: Description