def cubic_spline_breaksToknots(bvec):
    """
Given breakpoints generated from _cubic_spline_breaks,
[x0, x0, x0, x0, x1, x2, ..., xN-2, xf, xf, xf, xf],
return the spline knots [x0, x1, ..., xN-1=xf]. 
This function ``undoes" _cubic_spline_breaks:
knot_vec = _cubic_spline_breaks2knots(_cubic_spline_breaks(knot_vec))
    """
    return bvec[3:-3]
