Infinite loop but where?

object Prop {
  def simplify(prop : Prop) : Prop = {
    prop match {
      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 _ => {
        if (testSimp(prop)) simplify(prop)
        else prop
      }
    }
  }
  def testSimp (prop : Prop) : Boolean = {
    if (simplify(prop) == prop) false
    else true
  }
}