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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 _transform_col(self, x, i):
 """Encode one categorical column into labels. Args: x (pandas.Series):
 a categorical column to encode i (int):
 column index Returns: x (pandas.Series):
 a column with labels. """

        return x.fillna(NAN_INT).map(self.label_encoders[i]).fillna(0)