@staticmethod
def removable_cells(obj):
    """
        List object removable 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, 0, 1, 1], [1, 1, 0, 0]])
            sage: ParallelogramPolyominoGridViewAdapter.removable_cells(pp)
            [(1, 0), (0, 1)]
        """
    heights = [(0, 0)] + list(zip(obj.upper_heights(), obj.lower_heights()))
    heights.append((heights[-1][1],) * 2)
    cells = []
    for i in range(1, len(heights) - 1):
        x1, y1 = heights[i - 1]
        x2, y2 = heights[i]
        x3, y3 = heights[i + 1]
        if x2 + 1 < y2 and x2 != x3 and x2 + 1 < y1:
            cells.append((x2, i - 1))
        if x2 < y2 - 1 and y1 != y2 and y2 - 1 > x3:
            cells.append((y2 - 1, i - 1))
    if len(heights) > 3:
        x1, y1 = heights[-3]
        x2, y2 = heights[-2]
        if y1 < y2 or x2 + 1 == y2:
            cells.append((y2 - 1, len(heights) - 3))
    return cells
