The javadoc for ObjectOutputStream.writeUnshared and
ObjectInputStream.readUnshared states that writeUnshared/readUnshared cannot
be overridden in subclasses which use ObjectInputStream/ObjectOutputStream's
public one-argument constructor. This is not entirely accurate--such
overriding is allowed provided the subclass is constructed in a
security context possessing the "enableSubclassImplementation"
SerializablePermission.