person p; p.talk();                    
 empty


    p.become(Student); p.talk();           
 OOP


    p.become(Employer); p.talk();          
 $$


    p.become(new adult_person); p.talk();  
 interesting


    p.become(Student); p.talk();           
 OOP (new student)


    p.become(&p); p.talk();                
 $$ (old role)


    p.become(Person);  // initial state