Goal:
  
  A : Type
  B : Type
  R : nat → A → B → Type
  l : list A
  l' : list B
  a : All2i
        (λ (i : nat) (x : A) (y : B), nth_error l (i - 0) = Some x × R i x y)
        0 l l'
  i : nat
  x : A
  y : B
  X : nth_error l i = Some x × R i x y
  ============================
   (nth_error l i = Some x × R i x y)


DEBUG:Going to execute:
let do_atom := solve
           [ trivial
           with eq_true
           | reflexivity
           | symmetry; trivial
           | contradiction
           | congruence ]
           in
           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"