Draft: Resolve "Modify Proost's grammar" partially
Add dependent and non dependent product type to proost's grammar.
- 
(x :A) -> Bfor dependant products - 
A -> Bfor non-dependant ones, this one is the issue. 
Add dependent and non dependent product type to proost's grammar.
(x :A) -> B for dependant productsA -> B for non-dependant ones, this one is the issue.