[rqlrewrite] Enhance detection of need for Exists node
We actually want one only if this is not a (`And` / `Or`) binary tree of `Not` or
`Exists` nodes, so write a recursive function to tell so.
Related to #17074119
.. -*- coding: utf-8 -*-
Server Architecture
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.. image:: images/server-class-diagram.png
`Diagramme ArgoUML`_
[FIXME]
Make a downloadable source of zargo file.
.. _`Diagramme ArgoUML`: cubicweb.zargo