Skeleton

@densh could you please review the ideas for case studies?
Well, we have a problem there. The thing is in order to use @i? it has to be all or nothing, for now. So, in this case it is better to have only exp as the one. We can discuss on how to make it work for having all @i?. The problem is that for example: 
```
@i? def plus(lhs: Int @i?, rhs: Int @i?): Int @i? = lhs + rhs
```
Will result in a typing error if `rhs` is dynamic and `lhs` is static. Any ideas how to solve this?