SUBROUTINE KNFTPK(NTRK, ITRK, PMOM, VERTX, UPDATE, LCOVAR,
     *                  IK, IDTYP, CHISQ)
 
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
C  Build a new particle from several particles with a total momentum
C  constraint. Insert the new particle in the KN list.
C
C  NTRK      Integer variable (read)
C            # of input tracks
C
C  ITRK      Integer variable (read)
C            List of KN tracks.
C
C  PMOM      real variable (read)
C            Total momentum to constrain to
C
C  VERTX     real array (read)
C            (x,y,z) point at which to evaluate the mass constraint
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  LCOVAR    logical variable (read)
C            TRUE  ==> calculate covariance matrix
C            FALSE ==> do not calculate covariance matrix
C
C  IK        integer variable (read)
C            KN track number to write the fitted particle
C
C  IDTYP     integer variable (read)
C            Particle type to build
C
C *CHISQ     real variable (write)
C            Chisq of fit.
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>