sealed trait Expr
case class N(n: Int) extends Expr
case class Var(x: String, a: Int, n: Int) extends Expr
case class Add(n: Expr*) extends Expr
case class Mul(n: Expr*) extends Expr

def add(xs: List[Expr]): Expr = xs match {
  case List() => N(0)
  case List(xs) => xs
  case xs => Add(xs: _*) // Add(List(N(1), N(2), Var(x,2,2)))
}

println(add(List(N(1),N(2),Var("x",2,2))))