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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def keep_(self, *cols) -> "Ds": """ 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")`` """

        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