Parametric attributes

[Richard (Rikki) Andrew Cattermole]

The simplest solution to this is what I call contract invalidation.

When passing in a callback, that has less guarantees on it and does not 
escape, the function will match it.

Timon wants something a lot more expressive such an effect system, and 
after last time I discussed with him on it, I haven't bothered to 
continue down this path.

Realistically somebody needs to make a decision here and run with it.

Are we going effect system like with variability to attributes, or a 
simpler subset of it.