type
  TPerson = object of TObject
    name: string

  TStudent = object of TPerson
    id: int

method toString1(self: TPerson): string =
  return self.name

method toString1(self: TStudent): string =
   return self.name & " " & $self.id

method toString2(self: var TPerson): string =
  return self.name

method toString2(self: var TStudent): string =
  return self.name & " " & $self.id


var a: TPerson
a = TStudent(name: "Anton", id: 100)
assert(a.toString1() == "Anton 100") # not ok (id is a random number)
assert(a.toString2() == "Anton 100") # not ok (id is a random number)

var b: TStudent
b = TStudent(name: "Anton", id: 100)
assert(b.toString1() == "Anton 100") # ok
assert(b.toString2() == "Anton 100") # ok