SUBROUTINE KNFTXB(ISH1, ISH2, MASS, VERTX, LCOVAR, IK, CHISQ,
     *                  IDTYP)
 
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
C  Build a new neutral particle from 2 showers in XB list and add the new
C  particle to the KN list. The covariance matrix of the new particle is
C  computed.
C
C  ISH1      Integer variable (read)
C            Track #1 in XB list
C
C  ISH2      Integer variable (read)
C            Track #2 in XB list
C
C  MASS      real variable (read)
C            Mass to constrain to
C
C  VERTX     real array (read)
C            (x,y,z) point at which to evaluate the mass constraint
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 (51=pi0, 52=eta, etc.)
C
C *CHISQ     real variable (write)
C            Chisq of fit.
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>