def _curve(x1, y1, x2, y2, hunit = HUNIT, vunit = VUNIT):
    """
    Return a PyX curved path from (x1, y1) to (x2, y2),
    such that the slope at either end is zero.
    """
    ax1, ax2, axm = x1 * hunit, x2 * hunit, (x1 + x2) * hunit / 2
    ay1, ay2 = y1 * vunit, y2 * vunit
    return pyx.path.curve(ax1, ay1, axm, ay1, axm, ay2, ax2, ay2)