'Internal one step real root refinement procedure.'

def _inner_refine(self):

    if (self.mobius is None):
        return self
    (f, mobius) = dup_inner_refine_real_root(self.f, self.mobius, self.dom, steps=1, mobius=True)
    return RealInterval((mobius + (self.neg,)), f, self.dom)
