Goal:
  
  A : Type
  a : A
  l : list A
  IHl : ∀ (n : nat) (isdecl : n < #|l|),
          nth_error l n =
          Some (safe_nth l (exist (λ n0 : nat, n0 < #|l|) n isdecl))
  n' : nat
  isdecl : S n' < S #|l|
  ============================
   (nth_error l n' = nth_error l n')


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"