When you practise well your unskillful volitions and actions V(x)
gradually decrease until finally they drop below enlightenment level E
and your mind achieves nirvana:
Mind that is freed from greed, hatred and delusion doesn't grasp on anything, there's nothing that could remove it from that calm, stable equilibrium state.
So, the definite answer is: It is not possible to 'leave' Nirvana.