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"Making a contingency plan for an unknown future..." A 'plan' involving two agencies is a 'deal'!
I myself believe that a plan such as that supposedly suggested by Tory backbenchers - voluntarily putting no candidate up for election in one constituency in return for a quid pro quo in another - should virtually be treated as a criminal conspiracy.
Why? Because electors in both are denied an opportunity to cast a vote for the party they actually want in power. Naturally, it may often happen in the normal course of events that there is no candidate standing for the party an individual elector wishes to win power. (Yes, I know that technically we vote for individual candidates, but the party they represent is hardly an irrelevance!) However, deliberately CREATING such a situation for political gain should definitely be illegal.