Returns ((top, left), (bottom, right)) of bounding box

        A bounding box is the smallest rectangle that contains all selections.
        Non-specified boundaries are None.

def get_bbox(self):
        """Returns ((top, left), (bottom, right)) of bounding box

        A bounding box is the smallest rectangle that contains all selections.
        Non-specified boundaries are None.

        """

        bb_top, bb_left, bb_bottom, bb_right = [None] * 4

        # Block selections

        for top_left, bottom_right in zip(self.block_tl, self.block_br):
            top, left = top_left
            bottom, right = bottom_right

            if bb_top is None or bb_top > top:
                bb_top = top
            if bb_left is None or bb_left > left:
                bb_left = left
            if bb_bottom is None or bb_bottom < bottom:
                bb_bottom = bottom
            if bb_right is None or bb_right > right:
                bb_right = right

        # Row and column selections

        for row in self.rows:
            if bb_top is None or bb_top > row:
                bb_top = row
            if bb_bottom is None or bb_bottom < row:
                bb_bottom = row

        for col in self.cols:
            if bb_left is None or bb_left > col:
                bb_left = col
            if bb_right is None or bb_right < col:
                bb_right = col

        # Cell selections

        for cell in self.cells:
            cell_row, cell_col = cell

            if bb_top is None or bb_top > cell_row:
                bb_top = cell_row
            if bb_left is None or bb_left > cell_col:
                bb_left = cell_col
            if bb_bottom is None or bb_bottom < cell_row:
                bb_bottom = cell_row
            if bb_right is None or bb_right < cell_col:
                bb_right = cell_col

        if all(val is None for val in [bb_top, bb_left, bb_bottom, bb_right]):
            return None

        return ((bb_top, bb_left), (bb_bottom, bb_right))