def test_expand_excludes_terminals(self):
tree = ParseTree(self.A)
self.assertEqual(frontier_values(tree), [self.A])
tree.expand([self.B, "hello", self.B])
self.assertEqual(frontier_values(tree), [self.B, self.B])
tree.frontier[1].expand([self.A, "world", self.A])
self.assertEqual(frontier_values(tree), [self.B, self.A, self.A])
def test_expand_propagates_to_ancestors(self):
tree = ParseTree(self.A)
self.assertEqual(frontier_values(tree), [self.A])
tree.expand([self.B, self.B])
self.assertEqual(frontier_values(tree), [self.B, self.B])
tree.frontier[1].expand([self.A, self.B, self.A])
self.assertEqual(frontier_values(tree),
[self.B, self.A, self.B, self.A])
def test_expand_empty_production_shrinks_frontier(self):
tree = ParseTree(self.A)
self.assertEqual(frontier_values(tree), [self.A])
tree.expand(())
self.assertEqual(frontier_values(tree), [])
