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
    let H0 := CoreTactics.block in
    let H1 := 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:
tac H
DEBUG LTAC Evaluated term: (let H := CoreTactics.block in
                            let H0 := CoreTactics.block in
                            let H1 := 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')