subroutine kfit_vertex_known(ktrk, update_track, vtx0, Vvtx0,
     *                             chisq, error)
*
* begin_doc
*
*  Constrain a track to come from a 3-D vertex having a covariance matrix.
*  The vertex is not modified.
*
*  Input Parameters:
*  ktrk          integer variable
*                Track number to fit
*
*  update_track  integer variable
*                 0 ==> Calculate chisquare only (do not update track)
*                 1 ==> Calculate chisquare, update track
*                 2 ==> Calculate chisquare, update track & cov matrix
*
*  vtx0(3)       double precision array
*                Vertex point to constrain to
*
*  Vvtx0(3,3)    double precision array
*                Covariance matrix of vtx0(1-3)
*
*  Output Parameters:
*  chisq         double precision variable
*                Chisquare of fit
*
*  error         integer variable
*                 0 ==> All OK
*                >0 ==> Error
*
*  Other routines:
*
*  Notes:
*
*  Author:   Paul Avery      Created:  Tue Sep 23 21:58:44 EDT 1997
*
*  Major revisions:
*
*
* end_doc