functions in hex.i - r

             c= reg_track(x, y, z, rays, s)  
 
     track RAYS through regular mesh defined by the 1D coordinate  
     arrays X, Y, and Z.  Return values S and C are as for  
     hex5_track, where the mesh is numberof(X) by numberof(Y) by  
     numberof(Z).  

builtin function, documented at i0/hex.i   line 83