@staticmethod
def addable_cells(obj):
    """
        List object addable cells

        TESTS ::

            sage: from sage.combinat.parallelogram_polyomino import ParallelogramPolyomino
            sage: from sage_widget_adapters.combinat.parallelogram_polyomino_grid_view_adapter import ParallelogramPolyominoGridViewAdapter
            sage: pp = ParallelogramPolyomino([[0, 1, 0, 1], [1, 1, 0, 0]])
            sage: ParallelogramPolyominoGridViewAdapter.addable_cells(pp)
            [(1, 0), (2, 1), (1, 2)]
        """
    cells = []
    upper_heights = obj.upper_heights()
    for i, c in enumerate(upper_heights[1:]):
        if c != upper_heights[i]:
            cells.append((c - 1, i + 1))
    lower_heights = obj.lower_heights()
    for i, c in enumerate(lower_heights[1:]):
        if c != lower_heights[i]:
            cells.append((lower_heights[i], i))
    height, width = obj.geometry()
    cells += [(height, width - 1), (height - 1, width)]
    return cells
