SUBROUTINE KFVTX3(IT, UPDATE, Z, CHISQ, LERROR)
C
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
C     Refit a track to go through a fixed x,y,z point.
C
C  IT      integer variable (read)
C          Number of the track to be fit
C
C  UPDATE  integer variable (read)
C          0 ==> do not update input track parameters (just calculate chisq)
C          1 ==> update input track parameters only
C          2 ==> update input track parameters & covariance matrices and
C                replace the original unconstrainted parameters by the
C                new ones. ****** This is a dangerous option. ******
C
C  Z       real array (read)
C          Point through which the track must pass
C
C *CHISQ   real variable (write)
C          Chisquare of fit
C
C *LERROR  logical variable (write)
C          TRUE if error
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>