Trying to implement CL:MAPCAN in scala

Question
2

Perhaps it is the same problem as More help understanding the _ variable

because when I added additional implementations of list2 and list3, the compiler errors go away

  def list3(a:Int,b:Int,c:Int):List[Int] = {
    List(a,b,c)
  }
  def list2(a:Int,b:Int):List[Int] = {
    List(a,b)
  }