SUBROUTINE KWMASK(IK, MASS, UPDATE, CHISQ)
 
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
C  Constrain the mass of a particle in the KN list to be MASS. The track
C  parameters and their covariance matrix are recalculated.
C
C  IK        integer variable (read)
C            KN track number
C
C  MASS      real variable (read)
C            Mass to constrain to
C
C  UPDATE    integer variable (read)
C            0 ==> do not update track parameters (just calculate chisq)
C            1 ==> update track parameters and covariance matrix
C
C *CHISQ     real variable (write)
C            chisq of fit. Set to -1 if error.
C
C
C   The mass constraint is described in KWMAS
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>