"import generator" issue

when I read codes in password_guess.py, I found the code at the 50 line , "import generator",  is not resolved. I also can't find the module using pip or pypi index. Then I found the reference of the generator at the 2046 line, "generator.next_nodes_random_walk", so I think it's not third-party library. However, I'm not find the generator file in project also. So I wonder what's going on here. 