def ducrss(s1, s2):
    """
    Compute the unit vector parallel to the cross product of
    two 3-dimensional vectors and the derivative of this unit vector.

    http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/ducrss_c.html

    :param s1: Left hand state for cross product and derivative.
    :type s1: 6-Element Array of floats
    :param s2: Right hand state for cross product and derivative.
    :type s2: 6-Element Array of floats
    :return: Unit vector and derivative of the cross product.
    :rtype: 6-Element Array of floats
    """
    assert len(s1) is 6 and len(s2) is 6
    s1 = stypes.toDoubleVector(s1)
    s2 = stypes.toDoubleVector(s2)
    sout = stypes.emptyDoubleVector(6)
    libspice.ducrss_c(s1, s2, sout)
    return stypes.cVectorToPython(sout)