def add_vectors_xy(u, v):
    """Add two vectors, assuming they lie in the XY-plane.

    Parameters
    ----------
    u : sequence of float
        XY(Z) components of the first vector.
    v : sequence of float
        XY(Z) components of the second vector.

    Returns
    -------
    list
        Resulting vector in the XY-plane (Z = 0.0)

    Examples
    --------
    >>>

    """
    return [u[0] + v[0], u[1] + v[1], 0.0]
