def get_rowspanned_columns(self):
        """Return a dictionary mapping column indices to the number of columns
        spanned."""
        spanned_columns = {}
        current_row_index = self._index
        current_row_cols = sum(cell.colspan for cell in self)
        prev_rows = iter(reversed(self.section[:current_row_index]))
        while current_row_cols < self.section.num_columns:
            row = next(prev_rows)
            min_rowspan = current_row_index - int(row._index)
            if row.maximum_rowspan > min_rowspan:
                for cell in (c for c in row if c.rowspan > min_rowspan):
                    col_index = int(cell.column_index)
                    spanned_columns[col_index] = cell.colspan
                    current_row_cols += cell.colspan
        return spanned_columns
Return a dictionary mapping column indices to the number of columns
        spanned.