Goal:
    A : Type  B : Type  R : nat → A → B → Type  n : nat
  ============================
   (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 [] [])


DEBUG:Going to execute:
CoreTactics.intros_until_block
DEBUG LTAC Evaluated term: (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 [] [])