functions in rays.i - g

             get_s0(rays)  
 
     returns the s-coordinate of the point of closest approach of  
     the RAYS to the origin x=y=z=0.  The length of the first dimension  
     of RAYS may be either 3, 5, or 6; this first dimension will not  
     be present in the result.  
     The s-coordinate represents distance along the ray, increasing in  
     the direction the ray moves.  The 5 and 6 component ray coordinates  
     include a reference point (x,y,z) on the ray; s=0 at that point.  
     For the 3 component ray coordinate, get_s0 always returns 0.  

interpreted function, defined at i/rays.i   line 223