SUBROUTINE KVTXPT(NTRK, ITRK, UPDATE, Z0, CHISQ, LERROR)
C
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C  Forces several KN tracks to pass through a known space point.
C
C  NTRK    integer variable (read)
C          # of tracks to fit (length of ITRK)
C
C  ITRK    integer  array (read)
C          list of tracks to fit (length = NTRK)
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 Z0       real array (write)
C          (x,y,z) point through which all tracks must pass
C
C *CHISQ   real variable (write)
C          Chisquare of fit
C
C *LERROR  logical variable (write)
C          TRUE if error during vertex finding
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