Goal:
  
  A : Type
  P : A → Prop
  x : A
  l : list A
  H : P x
  HPl : Forall P l
  IHHPl : ∀ n : nat, nth_error l n = Some x → P x
  ============================
   (P x)


DEBUG:Going to execute:
let rec do_ccl :=
            try do_atom; repeat (do_intro; try do_atom); try arith_congr;
             (solve [ split; do_ccl ])
           in
           (solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"