def METHOD_NAME(self):
    class CustomTransformer(TreeForestTransformer):
        @handles_ambiguity
        def start(self, data):
            assert isinstance(data, list)
            assert len(data) == 4
            for tree in data:
                assert tree.data == 'start'
            return 'handled'
        @handles_ambiguity
        def ab(self, data):
            assert isinstance(data, list)
            assert len(data) == 1
            assert data[0].data == 'ab'
    tree = CustomTransformer(resolve_ambiguity=False).transform(self.forest)
    self.assertEqual(tree, 'handled')