Brexit

So there's a "gang of five" seeking to pressurise the Prime Minister to amend some of the details of the Brexit deal she is trying to get through Parliament.  But the problem is - this is a deal, an agreement - and as such is two-sided.  It cannot be altered unilaterally, and as far as I am aware, the EU have said that, as far as they are concerned, it is not negotiable.  Impasse?