def METHOD_NAME(self):
    r = MemoryRepo()
    base = make_commit()
    c1 = make_commit(parents=[base.id])
    c2a = make_commit(parents=[c1.id], message=b"2a")
    c2b = make_commit(parents=[c1.id], message=b"2b")
    r.object_store.add_objects([(base, None), (c1, None), (c2a, None), (c2b, None)])
    self.assertTrue(can_fast_forward(r, c1.id, c2a.id))
    self.assertTrue(can_fast_forward(r, c1.id, c2b.id))
    self.assertFalse(can_fast_forward(r, c2a.id, c2b.id))
    self.assertFalse(can_fast_forward(r, c2b.id, c2a.id))