http://interactivepython.org/runestone/static/pythonds/Graphs/Implementation.html
https://vevurka.github.io/dsp17/python/graphs_in_python/
https://www.programiz.com/python-programming/methods/built-in/iter
https://www.python.org/doc/essays/graphs/
http://www.bogotobogo.com/python/python_graph_data_structures.php