def test_valid_cells_from_bottom_edge(self):
grid = [
[1, 2, 4],
[3, 5, 6],
[7, 8, 9],
]
current_xy = (1,0)
self.assertEqual(8, value_at(grid, current_xy))
seen_cells = []
expected = [ (0, 0), (1, 1), (2, 0) ]
actual = valid_cells_from(current_xy, seen_cells, len(grid))
self.assertEqual(sorted(expected), sorted(actual))