SUBROUTINE KNSUMK(NTRK, ITRK, VERTX, IK, IDTYP)
 
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
C  Build a new particle with a covariance matrix from several particles by
C  adding their 4-vectors at a particular vertex. Add the resulting particle
C  to 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  VERTX     real array (read)
C            (x,y,z) point at which to evaluate the 4-vectors
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  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>