Set: pp::colors
  Set: pp::unicode
  Proved: T
theorem T (A : Type)
    (p : A → Bool)
    (f : A → A → A)
    (x y z : A)
    (H : p (f x x))
    (H::1 : x = y)
    (H::2 : x = z) : p (f y z) :=
    subst (subst H H::1) H::2