Goal:
  
  A : Type
  B : Type
  R : nat → A → B → Type
  n : nat
  l : list A
  l' : list B
  a : All2i R n l l'
  ============================
   (let H := CoreTactics.block in
    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:
lazymatch goal with
           | H:_ |- _ => tac H
           end