Scala coding dojo: Adding one to the first parameter of a case class, when that parameter is an integer

AddOne.scala

import shapeless._
import shapeless.ops.hlist.IsHCons

object ShapelessDojo {

  def addOneToCaseClass[C, H <: HList, E, T <: HList]
      (c: C)
      (implicit gen: Generic.Aux[C, H],
                h:   IsHCons.Aux[H, E, T],
                ev:  E =:= Int,
                ev2: (Int :: T) =:= H
      ): C = {

    val hList = gen.to(c)

    val elem = hList.head
    val tail = hList.tail

    val newElem = elem + 1

    gen.from(newElem :: tail)

  }
}