Ch 04

cc/ @fabioyamate @carlosgaldino.

Please review.
I didn't get what you mean by applying, but I could just use the `∨ ` operator in `ghci. Look:
![screen shot 2014-07-12 at 12 08 18 am](https://cloud.githubusercontent.com/assets/611891/3560495/e88fa798-0971-11e4-9c01-6580ee1bf878.png)