By doing:

```
@contextlib.contextmanager
def unlock(resource):
    resource = 'unlocked'
    ...
```
We don't really change  the `resource` that is passed in. Because Python is pass by object reference, doing a re-assignment operation does not actually change the `resource` i.e. `resource = 'unlocked'` does not really work. Furthermore, the `resource` variable, with which the `unlock` function is called, is a string; strings are immutable; thus, we need to pass in a mutable data type as the `resource` to be able to demonstrate the resource's "lock state" changing. While we could have used a simple "box" tactic i.e. `resource = ['locked']`, this seemed to distract from the goal of the example. Consequently, it seemed using a small class might be clearer without detracting from the example's value.

There is a small typo in metatable.py.

Should read `receives` rather than `recieves`.