*-- Author : S.Burke / J.V. Morris
      SUBROUTINE FKTRAN(DZ,Z1,S1,S2,D)
**********************************************************************                                        
*                                                                    *                                        
*  Transform track vector (S1) through DZ to (S2) in magnetic field. *                                        
*                                                                    *                                        
**********************************************************************                                        
*KEEP,FKNPL.                                                                                                  
*                                                                                                             
* Per-track values can go in H1WORK; note that LTRUE and LFIRST must                                          
* be set at least per event.                                                                                  
*                                                                                                             
* This is about 36k words long; the remaining common blocks are                                               
* about 3.6k in total. Some of this could be in /H1WORK/, but the                                             
* blocks would have to be reorganised.                                                                        
*                                                                                                             
* /FKPROJ/                                                                                                    
* /FKFILT/                                                                                                    
* /FKSMTH/                                                                                                    
* /FKINT/                                                                                                     
* /FKRSID/                                                                                                    
* /FKTRUE/                                                                                                    
* /FKDBG/                                                                                                     
*KEEP,FKINT.                                                                                                  
*KEND.                                                                                                        
*KEEP,FKPIDP.                                                                                                 
*KEND.                                                                                                        
**********************************************************************                                        
*                                                                                                             
* Thresholds for various approximations (*** UNOPTIMISED ***)                                                 
*                                                                                                             
* Max z step over which field is assumed constant                                                             
* If Dphi CALL GUFLD(R,B)                                                                               
* Work out Delta phi (don't re-order these lines!)                                                            
* Choose the order of approximation                                                                           
* Must consider change in field over DZ                                                                       
* Must take helix (x and y affected by dphi)                                                                  
* Ignore DPHI                                                                                                 
* Ignore change in phi for DX and DY                                                                          
* End of straight line approx                                                                                 
* Small angle approximations for cos and sin (to order DPHI**2)                                               
* Full expressions                                                                                            
* Remember values                                                                                             
* Change in x and y                                                                                           
* Some more abbreviations ...                                                                                 
* ... and the remaining differential coefficients                                                             
* Are corrections for Bx and By required?                                                                     
*                                                                                                             
* Apply 1st order corrections assuming that Bx and By are                                                     
* small but non-zero ........ (Bx,By,Bz) is still assumed                                                     
* to be constant across DZ ......                                                                             
*                                                                                                             
* If theta = 0, phi is undefined, so CON3 can be set to any value                                             
* Are corrections for field variation required?                                                               
* Get field at end point ...                                                                                  
      CALL GUFLD(R2,B2)                                                                                
*                                                                                                             
* DB is the mean difference from the assumed field along the trajectory,                                      
* assuming the field components vary linearly with Z over DZ.                                                 
* ie B and dB/dZ are non-zero but  d2B/dZ2  etc are assumed zero.                                             
*                                                                                                             
* Stop values getting too large (i.e. stop overflows)                                                         
*