@staticmethod
def removable_cells(obj):
    """
        List object removable cells

        TESTS ::

            sage: from sage.combinat.skew_partition import SkewPartition
            sage: from sage_widget_adapters.combinat.skew_partition_grid_view_adapter import SkewPartitionGridViewAdapter
            sage: SkewPartitionGridViewAdapter.removable_cells(SkewPartition([[4, 2, 1],[2, 1]]))
            [(0, 2), (1, 1), (2, 0), (0, 3)]
            sage: SkewPartitionGridViewAdapter.removable_cells(SkewPartition([[7, 4, 2, 1],[2, 1, 1]]))
            [(0, 2), (1, 1), (3, 0), (0, 6), (1, 3), (2, 1)]
        """
    ret = obj.inner().outside_corners()
    for c in obj.outer().corners():
        if not c in ret:
            ret.append(c)
    return ret
