[CLEAN] expression.py: self.__exp -> self.exp, because too much underscores kill my eyes. Added a bit of comments. Please note that the behavior has still not changed.
[CLEAN] expression.py: self.__exp -> self.exp, because too much underscores kill my eyes. Added a bit of comments. Please note that the behavior has still not changed.

akretion/openerp-server,akretion/openerp-server,akretion/openerp-server