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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 dateindex(self, col: str):
 """ Set a datetime index from a column :param col: column name where to index the date from :type col: str :example: ``ds.dateindex("mycol")`` """

        df = self._dateindex(col)
        if df is None:
            self.err("Can not create date index")
            return
        self.df = df
        self.ok("Added a datetime index from column", col)