Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  R' : ∀ (a : A) (b : B), R a b → Type
  x : A
  y : B
  l : list A
  l' : list B
  r : R x y
  rs : All2 R l l'
  r0 : R' x y r
  a0 : All2_dep R' rs
  ============================
   (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')