Section Demo.
Variable i : True.
Proof using i.
sleep time.
idtac.
sleep time.
(* Error, jump back to fix it, then Qed again *)
exact (i i).
Qed.
Proof using i.
sleep time.
idtac.
sleep time.
(* Here we use "a" *)
exact a.
Qed.

Lemma work_here : True /\ True.