This alias doesn't get involved _at all_ in that example expression.

`not` is a ruby keyword.  It's equivalent to `!` with slightly different precedence.  Generally using `not` does not involve any method calls, unless:

* You use an explicit receiver (e.g. `self.not`) or
* You are on 1.9+ and you have an object that has defined `!` as a method.

Your example spec expression works because in the spec context it is in, you have configured the matcher boolean operators.  If you remove your alias here you'll notice that the spec continues to pass.