*-- Author : S.Burke / J.V. Morris
      SUBROUTINE FKDIFF(S1,S2,S3)
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*                                                                    *                                        
*  S3 = S1 - S2 (takes care of phi range)                            *                                        
*                                                                    *                                        
*  Delta phi is in the range -pi to pi                               *                                        
*                                                                    *                                        
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*KEEP,FKPIDP.                                                                                                 
*KEND.                                                                                                        
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*