Goal:
    A : Type  n : nat  P : A → bool  d : A  H : is_true true
  ============================
   (S n < 0 → ∃ x : A, d = x ∧ P x)


DEBUG:Going to execute:
try do_atom
Level 2: Backtrace:

Proof is not complete.
Level 2: Backtrace:

No applicable tactic.
Level 1: evaluation returns
idtac of type tacvalue