Caculate the tangle vec of trace

[HandsomeDragon3]

Is it a mistake that s0 is used rather than s?
Can you explain it simply, thank you ~ @fangedward ~
// normal spline vector
double x0 = csp->calc_x(s0);
double y0 = csp->calc_y(s0);
double x1 = csp->calc_x(s0 + 2);
double y1 = csp->calc_y(s0 + 2);

[fangedward]

No, s0 is the current longitudinal position of the ego vehicle along the spline. Here we are calculating the normal vector at the current longitudinal position.