Goal:
  
  A : Type
  B : Type
  k : nat
  Q : nat → A → Type
  R : nat → A → B → Type
  l : list A
  l' : list B
  n : nat
  q : Alli Q n []
  ============================
   (let H := CoreTactics.block in
    All2i (λ (i : nat) (x : A) (y : B), Q i x × R i x y) n [] [])


DEBUG:Going to execute:
change (let _ := CoreTactics.block in T)