def testPreexistingLetters(self):
tiles = [None, None, None, 'x', 'z', 's', None, 'f', None]
nodes = self.create_vertical_nodes(tiles)
placements = Placement.placements('safe', nodes[5])
self.assertEqual(len(placements), 2)
placement = placements[1]
self.assertTrue(placement, Collision.safe(placement))
# The preexisting letters should contain the lettera 's' and 'f' because
# when placing the word 'safe' across from the letter 's' the letter 'f'
# would be used as well.
letters = Collision.preexisting_letters(placement)
self.assertEqual(len(letters), 2)
self.assertEqual(letters[0], 's')
self.assertEqual(letters[1], 'f')
