- https://gist.github.com/kentfredric/9285653

- https://gist.github.com/anonymous/55bcf30acab99308714b
# a + b = o
- https://gist.github.com/anonymous/0544b465c57f01f8c4a6
- https://gist.github.com/ingydotnet/bf92949734b6a23f2af6
- https://gist.github.com/ingydotnet/a61b3cd1c422a17c6e5d
