class actor { 
   actor  
  
  public static final int Person = 0;
  public static final int Student = 1;
  public static final int Employer = 2;
  public static final int Final = 3;
  
  public void walk() { if (exists()) self().walk(); }
  public void talk() { if (exists()) self().talk(); }
  public void think() { if (exists()) self().think(); }
  public void act() { if (exists()) self().act(); }
  
  public boolean exists() { return false; }
  public actor self() {  return this; }
  
  public void become(actor A) { }
  public void become(int R) { }
  };