abstract class Tree {

    case class Sum(left : Tree, right : Tree) extends Tree;
    case class Var(n : String) extends Tree;
    case class Const(v : Int) extends Tree;
    type Enviroment = String => Int;
    def eval(tree : Tree, env : Enviroment) : Int tree = match {
    case Sum(left, right) => eval(left, env) + eval(right, env)
    case Var(n) => env(n)
    case Const(v) => v
    }
}