Incorrect handling of predicates without base types

[codyroux]

The following predicate

inductive NoBase : Nat → Prop where
| succ : NoBase n → NoBase (n + 1)

derive_generator ∃ n, NoBase n

gives the error:

parenthesize: uncaught backtrack exception

which is quite confusing, especially in the absence of any syntax to point to.

I'm not sure what the right fix is here, ideally, we'd produce a correct (but useless) generator with a warning, but perhaps there's something simpler to implement.