# parse LDAP schema

		schema = ldap.schema.SubSchema(self.lo.lo.lo.read_subschemasubentry_s(self.lo.lo.lo.search_subschemasubentry_s()), 0)

		newattr = ldap.cidict.cidict(_MergedAttributes(self, ml).get_attributes())

		ocs_afterwards = set(newattr.get('objectClass', [])) - object_classes_to_remove

		# make sure we still have a structural object class

		if not schema.get_structural_oc(ocs_afterwards):

			structural_ocs = schema.get_structural_oc(object_classes_to_remove)

			if structural_ocs:

				univention.debug.debug(univention.debug.ADMIN, univention.debug.WARN, 'Preventing to remove last structural object class %r' % (structural_ocs,))

				object_classes_to_remove -= set(schema.get_obj(ldap.schema.models.ObjectClass, structural_ocs).names)

				ocs_afterwards = set(newattr.get('objectClass', [])) - object_classes_to_remove

			else:

				univention.debug.debug(univention.debug.ADMIN, univention.debug.ERROR, 'missing structural object class. Modify will fail.')

		# validate removal of object classes

		do_removal = True

		must, may = schema.attribute_types(ocs_afterwards)

		must = ldap.cidict.cidict(dict((x, x) for x in list(chain.from_iterable(x.names for x in must.values()))))

		may = ldap.cidict.cidict(dict((x, x) for x in list(chain.from_iterable(x.names for x in may.values()))))

		for attr in must.keys():

			if not newattr.get(attr):

				univention.debug.debug(univention.debug.ADMIN, univention.debug.INFO, 'The attribute %r is required in the current object classes.' % (attr,))

				do_removal = False

		for attr, val in newattr.items():

			if val and not must.get(attr) and not may.get(attr):

				univention.debug.debug(univention.debug.ADMIN, univention.debug.INFO, 'The attribute %r has is now known by any object class.' % (attr,))

				do_removal = False

		if do_removal:

			ml = [x for x in ml if x[0].lower() != 'objectClass'.lower()]

			ml.append(('objectClass', self.oldattr.get('objectClass', []), list(ocs - object_classes_to_remove)))