Allow Cf function to be redeclared

When I switch from one Haskell file to another in Vim I get this error:

![screen shot 2014-08-23 at 10 51 09 am](https://cloud.githubusercontent.com/assets/911911/4021325/249fec82-2aee-11e4-8438-2ed549fafe33.png)

This pull request just follows those instructions and it seems to fix the problem on my machine.