Attempt at implementing `++` for a statically sized list

NList.scala

object NList {

  import Nat._

  sealed trait NList[L <: Nat, A] {
    def ++[B >: A, M <: Nat](other: NList[M, B])
                             (implicit sum: Plus[L, M]): NList[sum.Out, B]
  }
  
  case object NNil extends NList[Zero, Nothing] {
    /*
    [error]  found   : NList.NList[M,B]
    [error]  required: NList.NList[sum.Out,B]
    [error]                              (implicit sum: Plus[Zero, M]): NList[sum.Out, B] = other
    [error]                                                                                 ^
    */
    def ++[B >: Nothing, M <: Nat](other: NList[M, B])
                             (implicit sum: Plus[Zero, M]): NList[sum.Out, B] = other
  }

}