Goal:
    A : Type  P : list A → A → Type
  ============================
   (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