He actually has allowed several that aspire to do so. But there is no way in which a vote in the commons can change the backstop so that there's an end date. It's a bit like having a vote to determine that the earth is flat, or the capital of Italy is Venice.
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One interesting point that has been entirely ignored by commentators. If Theresa May's Deal we’re somehow to win the vote today, the DUP has said it would abandon its agreement to support the governmen. That would mean that it wouldn't have any majority in Parliament, and would be liable to find it impossible to get anything passed - including the legislation needed for the deal to take effect.