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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 fill_nan(self, val: str, *cols):
 """ Fill NaN values with new values in the main dataframe :param val: new value :type val: str :param \*cols: names of the colums :type \*cols: str, at least one :example: ``ds.fill_nan("new value", "mycol1", "mycol2")`` """

        df = self._fill_nan(val, *cols)
        if df is not None:
            self.df = df
        else:
            self.err("Can not fill nan values")