functions in rays.i - d

             dirt_rays(rays)  
 
     returns 3-element (x,y,theta) representation of RAYS.  
     The first dimension of RAYS may be length 3, 5, or 6 to represent  
     the ray(s) in TDG/DIRT coordinates (x,y,theta), "best" coordinates  
     (x,y,z,theta,phi), or internal coordinates (cos,sin,y,z,x,r),  
     respectively.  The first dimension of the result always has length 3.  
     The TDG/DIRT coordinate system is based on the coordinates (x,y)  
     in a plane normal to the ray.  Unfortunately, the old TDG and DIRT  
     codes used an angle theta which has the opposite sense from the  
     "best" and internal coordinates.  Therefore, conversion from  
     TDG/DIRT coordinates to internal coordinates will reverse the  
     sign of theta.  Conversion from TDG/DIRT coordinates to "best"  
     coordinates always results in positive theta, but the angle phi  
     will be pi for positive input theta.  
     The slimits parameter -- used to specify the points along  
     a ray where the transport integration starts and stops -- is  
     measured from the point of closest approach of the ray described  
     by (x,y,theta) to the origin x=y=z=0.  Therefore, slimits is  
     independent of the TDG/DIRT ray coordinate representation.  
interpreted function, defined at i/rays.i   line 97