def to_rational(s):
    """Convert a raw mpf to a rational number. Return integers (p, q)
    such that s = p/q exactly."""
    sign, man, exp, bc = s
    if sign:
        man = -man
    if bc == -1:
        raise ValueError("cannot convert %s to a rational number" % man)
    if exp >= 0:
        return man * (1<<exp), 1
    else:
        return man, 1<<(-exp)