Function app

crates/kernel/src/inductive.rs:4535–4537

Helper: build `App(f, a)`

(f: AE, a: AE)

    }
    match self.try_get_const(&ctors[0])? {
      Some(KConst::Ctor { fields, .. }) => Ok(fields == 0),
      _ => Ok(false),
    }
  }
}