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:
reflexivity
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.