trait TFn1B {
  type In
  type Out
  type Apply[T <: In] <: Out
}

trait TFn1[I, O] extends TFn1B {
  type In = I
  type Out = O
}

trait >>[F1 <: TFn1[_, _], F2 <: TFn1[_, _]] extends TFn1[F1#In, F2#Out] {
  type Apply[T] = F2#Apply[F1#Apply[T]]
}