def _n2(a, b):
    """Return (a - b).evalf(2) if a and b are comparable, else None.
    This should only be used when a and b are already sympified.
    """
    # /!\ it is very important (see issue 8245) not to
    # use a re-evaluated number in the calculation of dif
    if a.is_comparable and b.is_comparable:
        dif = (a - b).evalf(2)
        if dif.is_comparable:
            return dif