SUBROUTINE KVRTX2(NTRK, ITRK, UPDATE, LUSEZ, Z, VZ, CHISQ, LERROR)
C
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C  Performs a vertex fit to several tracks in the KN list using only the
C  constraint that the tracks must intersect at a common x-y point. The
C  z value is not changed.
C
C  NTRK    integer variable (read)
C          # of tracks to fit (length of ITRK)
C
C  ITRK    integer  array (read)
C          list of tracks in KN list 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  LUSEZ   logical variable (read)
C          TRUE  ==> use the value of Z as the starting value
C          FALSE ==> compute the starting value with a straight line approx.
C
C *Z       real array (read/write)
C          On input,  initial values of (x,y,z)
C          On output, updated values of (x,y,z). Z(3) is not changed.
C
C *VZ      real array (write)
C          3x3 covariance matrix of z(1-3). Only the 2x2 piece is meaningful.
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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