func main_old()
{
  // cannot construct database directly
  //let mydb = SingletonDatabase()

  let db = SingletonDatabase.instance
  var city = "Tokyo"
  print("\(city) has population \(db.getPopulation(name: city))")

  let db2 = SingletonDatabase.instance
  city = "Manila"
  print("\(city) has population \(db.getPopulation(name: city))")
  print("SingletonDatabase has \(SingletonDatabase.instanceCount) instance(s)")
}