Goal:
  
  A : Type
  B : Type
  C : Type
  R : A → B → C → Type
  x : A
  y : B
  z : C
  l : list A
  l' : list B
  l'' : list C
  r : R x y z
  a : All3 R l l' l''
  ============================
   (eq_refl = eq_refl)


DEBUG:Going to execute:
match goal with
           | |- ?x = ?y =>
                 let x' := eval hnf in x in
                 let y' := eval hnf in y in
                 change_no_check (x' = y')
           end