@Override
    public Proto getProto()
    {
        Proto p = proto;
        if (p == null)
            proto = p = parentMessage.getProto();
        return p;
    }
Represents an extend block declared in either the {@link Proto} or nested in a {@link Message}.

@author Philippe Laflamme
protostuff-parser/src/main/java/io/protostuff/parser/Extension.java