Proposal to solve the `EitherT` problem.

[Henning Thielemann]

Am 13.08.2013 20:31, schrieb Gabriel Gonzalez:
> I'm still in favor of adding `EitherT` to `transformers` in addition to
> `ErrorT`.  The only person who disagreed the last discussion was Ross,
> so it's a matter of convincing him.

I also prefer Ross' suggestion of Except/ExceptT.

Using Either as a monad for me is abuse, as is defining Monad for pairs, 
Num for functions, Ord for complex numbers etc.