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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 index(self, col: str):
 """ Set an index to the main dataframe :param col: column name where to index from :type col: str :example: ``ds.index("mycol")`` """

        df = self._index(col)
        if df is None:
            self.err("Can not create index")
            return
        self.df = df