code: def _at(cls, verb):
          
          # Named (listed) columns are always included
          columns = cls.select(verb)
          final_columns_set = set(cls.select(verb))
          groups_set = set(_get_groups(verb))
          final_columns_set -= groups_set - set(verb.names)
  
          def pred(col):
              if col not in verb.data:
                  raise KeyError(
                      "Unknown column name, {!r}".format(col))
              return col in final_columns_set
  
          return [col for col in columns if pred(col)]
docs: A verb with a select text match