d in
   <a class="xref" href="biblio.html#STON90A">[ston90a]</a>, was released to a few external users in
   June 1989.  In response to a critique of the first rule system
   (<a class="xref" href="biblio.html#STON89">[ston89]</a>), the rule system was redesigned (<a class="xref" href="biblio.html#STON90B">[ston90b]</a>), and Version 2 was released in June 1990 with
   the new rule system.  Version 3 appeared in 1991 and added support
   for multiple storage managers, an improved query executor, and a
   re