Set: pp::colors
  Set: pp::unicode
  Set: lean::pp::implicit
  Set: pp::colors
  Assumed: N
  Defined: T
Definition T (a : N) (f : N → N) (H : f a == a) : f (f a) == f (f a) :=
    @SubstP N (f a) a (λ x : N, f (f a) == f (f a)) (@Refl N (f (f a))) H