def previous(self, rows: List[Row]) -> List[Row]:
        """
        Takes an expression that evaluates to a single row, and returns the row that occurs before
        the input row in the original set of rows. If the input row happens to be the top row, we
        will return an empty list.
        """
        if not rows:
            return []
        input_row_index = self._get_row_index(rows[0])
        if input_row_index > 0:
            return [self.table_data[input_row_index - 1]]
        return []
Takes an expression that evaluates to a single row, and returns the row that occurs before
        the input row in the original set of rows. If the input row happens to be the top row, we
        will return an empty list.