abstract class Expr { 
  def isNumber: Boolean 
  def isSum: Boolean 
  def numValue: Int 
  def leftOp: Expr 
  def rightOp: Expr 
} 

class Number(n: Int) extends Expr { 
  def isNumber: Boolean = true 
  def isSum: Boolean = false 
  def numValue: Int = n 
  def leftOp: Expr = error("Number.leftOp") 
  def rightOp: Expr = error("Number.rightOp") 
} 

class Sum(e1: Expr, e2: Expr) extends Expr { 
  def isNumber: Boolean = false 
  def isSum: Boolean = true
  def numValue: Int = error("Sum.numValue") 
  def leftOp: Expr = e1 
  def rightOp: Expr = e2 
} 


def eval(e: Expr): Int = { 
  if (e.isNumber) e.numValue 
  else if (e.isSum) eval(e.leftOp) + eval(e.rightOp) 
  else error("unrecognized expression kind") 
} 


// try eval(  new Sum(new Number(1), new Sum(new Number(3), new Number(7))) )


def printExpr(e: Expr) { 
  if (e.isNumber) print(e.numValue) 
  else if (e.isSum) { 
    print("(") 
    printExpr(e.leftOp) 
    print("+") 
    printExpr(e.rightOp) 
    print(")") 
  } else error("unrecognized expression kind") 
}