Simplified definition of Modifiable

This simplifies a lot of the code and makes the logic easier to follow.

I think that the Eq instance is a bit of a hack and it would be better to have a data type which differentiates the different styles but you might prefer it like this.

Modifier might just be able to be a type synonym as well.

Also, Line 68-82, what is the difference between the two implementations?

I have a flight in a few hours so it's not very clean.