close #511810: bad rql generated when looking for vocabulary for a relation on an entity which doesn't exist (yet)
technical details:
* add a graph of linked variables in the RRQLExpression instance
* modify rql rewriter to remove relations where an unexistant
variable and or a variable linked to an unexistant variable is used.
* activate this feature in Entity.unrelated_rql if the entity doesn't exists yet
""" CW - nevow/twisted client
:organization: Logilab
:copyright: 2001-2009 LOGILAB S.A. (Paris, FRANCE), license is LGPL v2.
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:license: GNU Lesser General Public License, v2.1 - http://www.gnu.org/licenses
"""