import java.util.*;
public class Kindergarden implements Iterable<Child>{

    Set<Child> children = new HashSet<Child>();
    
    public Iterator<Child> iterator(){
        return children.iterator();
    }
    public void enroll(Child c){
        children.add( c );
    }
    
    public void withdraw(Child c){
        children.remove(c);
    }
    
    
}