Goal:
  
  A : Type
  B : Type
  P : nat → A → B → Type
  x : A
  y : B
  l : list A
  l' : list B
  r : P #|l| x y
  X : All2i_len P l l'
  IHX : All2i_len (λ n : nat, P (#|l| - S n)) (List.rev l) (List.rev l')
  n : nat
  x0 : A
  y0 : B
  X0 : P (#|l| - S n) x0 y0
  ============================
   (P (#|l| - S n) x0 y0)


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"