Goal:
  
  A : Type
  B : Type
  R : nat → A → B → Type
  n : nat
  l : list A
  l' : list B
  a : All2i R n l l'
  ============================
   (All2i
      (λ (i : nat) (x : A) (y : B),
         (if i <? n then False else nth_error l (i - n) = Some x) × R i x y)
      n l l')


DEBUG:Going to execute:
fun tac H =>
             try intros until H; intro_block H;
              (try simpl in H; DepElim.simplify_equations_in H);
              DepElim.generalize_by_eqs_vars H; CoreTactics.block_goal; 
              tac H; CoreTactics.intros_until_block; DepElim.simpl_dep_elim;
              CoreTactics.unblock_goal