Goal:
  
  A : Type
  B : Type
  R : nat → A → B → Type
  n : nat
  x : A
  y : B
  l : list A
  r : list B
  r0 : R n x y
  a : All2i R (S n) l r
  ============================
   (eq_refl = eq_refl)


DEBUG:Going to execute:
let x' := eval hnf in x in
           let y' := eval hnf in y in
           change_no_check (x' = y')