code: def keep_(self, *cols) -> "Ds":
          
          try:
              ds2 = self._duplicate_(self.df[list(cols)])
          except Exception as e:
              self.err(e, "Can not remove colums")
              return
          self.ok("Columns", " ,".join(cols), "kept")
          return ds2
docs: Returns a dataswim instance with a dataframe limited
          to some columns
  
          :param cols: names of the columns
          :type cols: str
          :return: a dataswim instance
          :rtype: Ds
  
          :example: ``ds2 = ds.keep_("Col 1", "Col 2")``