ggers.
     It modifies queries to take rules into consideration, and then
     passes the modified query to the query planner for planning and
     execution.  It is very powerful, and can be used for many things
     such as query language procedures, views, and versions.  The
     theoretical foundations and the power of this rule system are
     also discussed in <A
HREF="biblio.html#STON90B"
><I
><A
HREF="http://db.cs.berkeley.edu/papers/ERL-M90-36.pdf"
TARGET="_top"
>    On Rules, Procedures, Caching 