def crossproduct(a, b):
    """Return cross product (a X b)

    Parameters
    ----------
    a : 3-element list of floats
        first set of values in cross-product calculation

    b : 3-element list of floats
        second set of values in cross-product calculation

    Returns
    -------
    c :  3-element list of floats
        result of cross-product calculation
    """
    c = [0]*3
    c[0] = a[1]*b[2] - a[2]*b[1]
    c[1] = a[2]*b[0] - a[0]*b[2]
    c[2] = a[0]*b[1] - a[1]*b[0]
    return c