}
                public void onIdentifier(IdentifierNd id) {                    ObjectMap om = getMap(id.getParent().getParent());                    ObjectType ot = om.getObjectType();                    Role role = (Role) ot.getProperty(id.getName());                    if (role == null) {                        m_errors.fatal                            (id, "no such property: " + id.getName());                    } else {                        if (role.isCollection()) {                            m_errors.fatal                                (id, "collections cannot be keys: " +                                 id.getName());                        } else {                            om.getKeyProperties().add(role);                            role.setNullable(false);                        }                    }                }