'Compute a**(-1), if possible.'

def revert(self, a):

    I = (self.ring.ideal(a.data) + self.base_ideal)
    try:
        return self(I.in_terms_of_generators(1)[0])
    except ValueError:
        raise NotReversible(('%s    not    a    unit    in    %r' % (a, self)))
