Goal:
    A : Type  B : Type  R : nat → A → B → Type  n : nat
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    All2i
      (λ (i : nat) (x : A) (y : B),
         (if i <? n then False else nth_error [] (i - n) = Some x) × R i x y)
      n [] [])


DEBUG:Going to execute:
change T; CoreTactics.intros_until_block
Level 3: T
 has value 
(let H := CoreTactics.block in
 All2i
   (λ (i : nat) (x : A) (y : B),
      (if i <? n then False else nth_error [] (i - n) = Some x) × R i x y) n
   [] [])