Finding out type of prepend.Out

prepend-test.scala

case class Record[A](userId: UUID, createdAt: LocalDateTime, version: Long, data: A)

type MetaData = UUID :: LocalDateTime :: Long :: HNil

def length[R <: HList](record: Record[A])
        (implicit gen: Generic.Aux[A, R],
         prepend: Prepend[R, MetaData]) = {
  val data = gen.to(record.data)
  val meta = record.userId :: record.createdAt :: record.version :: HNil
  val toWrite = data ::: meta

  // next line fails, missing Length[prepend.Out], but prepend.Out cannot be used in the implicit arg list.
  // What should be an alternate, but valid type signature of prepend.Out?
  val natLen = toWrite.length
                
  // Not gotten here, but probably yet another signature issue in waiting.
  Nat.toInt(natLen)
          
}