SUBROUTINE KVDIFF(Z1, VZ1, Z2, VZ2, CHISQ)
C
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
C     Finds the chisquare for 2 vertices to be the same.
C
C   Note: CHISQ is set to -1 if the routine cannot evaluate the combined
C         covariance matrix.
C
C  Z1      real array (read)
C          (x,y,z) of first vertex
C
C  VZ1     real array (read)
C          3x3 covariance matrix of first vertex
C
C  Z2      real array (read)
C          (x,y,z) of second vertex
C
C  VZ2     real array (read)
C          3x3 covariance matrix of second vertex
C
C *CHISQ   real variable (write)
C          Chisquare of fit
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>