title: schema and database evolution in the o2 object database system fabrizio ferrandina , guy ferran , thorsten meyer , roberto zicari , joëlle madec. authors: nan. venue: vldb. year: 1995.0
title: the trigs active object-oriented database system - an overview. authors: g. kappel , w. retschitzegger. venue: acm sigmod record. year: 1998.0