def complementary_iof(principal, complementary_iof_fee=0.0038):
    """Complementary IOF tax over the principal.

    If :math:`s` is the principal and :math:`I^{**}` is the complementary IOF
    fee, then the due complementary IOF tax over the principal is

    .. math::

        s\\ I^{**}.

    Parameters
    ----------
    principal: float, required
        The loan principal, which is the basis for the complementary IOF tax.
    complementary_iof_fee: float, required
        Aliquot applied over the principal. Its value is defined by law.
    """
    return float(principal * complementary_iof_fee)
