38 - lexicon should be session-specific

[joshsh]

There is a need for a "query session" object which assumes some of the
responsibilities of QueryEngine.  Certain applications, such as the
command-line interface, are not stateless and should not be treated as
such.  For instance, namespace prefixes defined in a session should retain
the same values for the duration of the session.  Currently, if another
thread or process were to redefine a namespace prefix in the course of the
session, it could corrupt subsequent queries and program definitions w.r.t.
their intended meaning.

Original link: http://code.google.com/p/ripple/issues/detail?id=38