Commit 5d943ab

| [memento](memento.py) | generate an opaque token that can be used to go back to a previous state |

| [observer](observer.py) | provide a callback for notification of events/changes to data |

| [publish_subscribe](publish_subscribe.py) | a source syndicates events/data to 0+ registered listeners |

| [specification](specification.py) | business rules can be recombined by chaining the business rules together using boolean logic |

| [state](state.py) | logic is organized into a discrete number of potential states and the next state that can be transitioned to |

| [strategy](strategy.py) | selectable operations over the same data |