def shiftxy(posx=0, posy=0):
    """shiftxy(posx=0, posy=0) -> stereo

    Return a two-dimensional stereo shift. If posy is zero (or omitted),
    this is equivalent to shift(posx).
    """
    if posx == 0 and posy == 0:
        return ()
    if posy == 0:
        return 1.0, float(posx)
    return 1.0, float(posx), 1.0, float(posy)
