unapply-instance.markdown

Suppose we want to lift an OptionT[List, Int] into a ReaderT[OptionT[List, _], String, Int]. The nice way (liftReaderT) currently doesn't work:

scala> List(1, 2, 3).liftM[OptionT].liftReaderT[String]
<console>:14: error: could not find implicit value for parameter F0: scalaz.UnapplyCo[scalaz.Monad,scalaz.OptionT[List,Int]]
              List(1, 2, 3).liftM[OptionT].liftReaderT[String]
                                 ^
<console>:14: error: value liftReaderT is not a member of scalaz.OptionT[List,Int]
              List(1, 2, 3).liftM[OptionT].liftReaderT[String]
                                           ^

Adding this UnapplyCo instance makes everything okay:

implicit def unapplyMFA1[TC[_[_]], F[+_], M0[F[+_], +_], A0](
  implicit TC0: TC[({ type L[x] = M0[F, x] })#L]
): UnapplyCo[TC, M0[F, A0]] {
  type M[+X] = M0[F, X]
  type A = A0
} = new UnapplyCo[TC, M0[F, A0]] {
  type M[+X] = M0[F, X]
  type A = A0
  def TC = TC0
  def leibniz = Leibniz.refl
}

I don't see any problem here. Is there a reason this isn't included, when apparently more complex instances (like the one provided by unapplyMFAB1) are?