And they also said they would fast-track an application of the UK, if they applied for membership after leaving.
But I guess, from the EU's perspective, you would need approval of all member states for both, which is incredibly hard to get these days.
And the current UK government probably does not see any of these 2 as an option...
http://www.bbc.co.uk/news/uk-politics-39291512 seems to give a good spread of opinions, including:
> "They might try to extract a political price," Lord Kerr said, "but legally they couldn't insist that you leave."
But I could be wrong ;)
I thought it could be stopped at anytime with agreement of all member states. According to Lord Kerr (who devised Article 50):
"It is not irrevocable. You can change your mind while the process is going on.
During that period, if a country were to decide actually we don't want to leave after all, everybody would be very cross about it being a waste of time.
They might try to extract a political price but legally they couldn't insist that you leave."
ANYTHING can be done with agreement of all member states. Including completely rewriting the Treaty of Rome, Maastricht Treaty, and Treaty of Lisbon along with every other agreement that makes up the EU.