Scala problem

/* Test score: 5/6
 *
 * Test testSimplify1 failed:
 * Expected Or(Or(Or(And(Atom(x1),And(Not(Atom(x2)),Not(Atom(x3)))),Impl(And(Atom(x3),Atom(x4)),Atom(x1))),Equiv(Atom(x2),Or(Atom(x1),Atom(x2)))),Or(Not(Atom(x4)),Or(Not(Atom(x3)),Not(Atom(x4)))))
 * but got Or(Or(Or(And(Atom(x1),And(Not(Atom(x2)),Not(Atom(x3)))),Impl(Not(Or(Not(Atom(x3)),Not(Atom(x4)))),Atom(x1))),Equiv(Atom(x2),Or(Atom(x1),Atom(x2)))),Or(Not(Atom(x4)),Not(And(Atom(x3),Atom(x4)))))
 */
//--------------------------------------------
abstract case class Prop

case class True() extends Prop
case class False() extends Prop
case class Atom(s : String) extends Prop
case class Not(Arg1 : Prop) extends Prop
case class And(Arg1 : Prop, Arg2 : Prop) extends Prop
case class Or(Arg1 : Prop, Arg2 : Prop) extends Prop
case class Impl(Arg1 : Prop, Arg2 : Prop) extends Prop
case class Equiv(Arg1 : Prop, Arg2 : Prop) extends Prop

object Prop {
  /*def simplify(prop : Prop) : Prop = {
    prop match {
      case Not(And(Not(a), Not(b))) => Or(simplify(a), simplify(b))
      case Not(Or(Not(a), Not(b))) => And(simplify(a), simplify(b))
      case Not(Or(a,b)) => simplify(And(Not(a),Not(b)))
      case Not(And(a,b)) => simplify(Or(Not(a),Not(b)))
      case Not(Not(a)) => simplify(a)
      case _ => prop
    }
  }*/
  val deMorganAndDoubleNegation: Prop => Prop = {
    case Not(Or(a, b)) => And(Not(a), Not(b))
    case Not(And(a, b)) => Or(Not(a), Not(b))
    case Not(Not(a)) => a
    case a => a
  }

  val simplify: Prop => Prop = deMorganAndDoubleNegation andThen {
    case And(a, b) => And(simplify(a), simplify(b))
    case Or(a, b) => Or(simplify(a), simplify(b))
    case Not(a) => Not(simplify(a))
    case a => a
  }
}