XSB Src (SVN)-old

Previous commit had bug. Set ep of old usermod psc record only if it
is undefined.

Changed slightly handling of exported predicates when loading a module.
The new psc record for an exported predicate has, for a long time, been
added to usermod, so that after consulting a module, the user can directly
call it from the top-level. But if there previously was a psc record
for the same-named predicate already in usermod, it would be lost, and so
calls to "that" psc record will not be found. So I changed it so that
old usermod psc record now branches to the ep of the newly added psc record
so it works as expected. I used the import_as mechanism to get this done.

Forgot to add.

Added documentation of new library prolog_db.

Added prolog_db.P to be compiled.

A new set of routines that support creating and using "Prolog Databases"
in XSB. A Prolog DB is Prolog term that represents a set of rules. Users
can create sets of rules, pass them aroung, and call goals in them.
These Prolog DBs are represented as a Prolog term which implements a trie,
with hash tables at each branch point. They are asymptotically quite
efficient. See documentation in User Manual 2 under Libraries.

Added documentation on interning terms and using (e.g.)
:- table p/2 as intern.

Updated for release of 3.5.

Some extra minor changes.

Major rewrite of incremental tabling system; updated biblio references
and credits.