総称型の抜け穴

import java.util.*;

class GenericTest
{
    public static void main(String[] argv)
    {
        List<Exception> a = new ArrayList<Exception>();
        List<? extends Exception> b = a;
        List c = b;
        c.add(new Throwable());
        for(Exception d : a) System.out.println(d.toString());
    }
}