package foo

class A() {
    var c = 3
}

fun A.i() : Int {
    c = c + 1
  return c
}

fun box() : String {
    var a1 : A? = A()
    var a2 : A? = null
  if (a1?.i() != 4) {
    return "1";
  }
    if (a1?.c != 4) {
      return "2";
    }
  if (a2?.c != null) {
      return "3";
  }
    a2?.i()
  if (a1?.c != 4) {
        return "4";
  }
    a2 = a1
  if (a2?.i() != 5) {
      return "5";
  }
  if ((a2?.i() != 6) || (a1?.c != 6)) {
    return "6"
  }
   return "OK"
}