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


DEBUG:Going to execute:
try do_atom; repeat (do_intro; try do_atom); try arith_congr;
            (solve [ split; do_ccl ])