subroutine kvtx_unknown(ntrk, list_trk, update_tracks, luse_vtx,
     *                        vtx, Vvtx, chisq, error)
*
* begin_doc
*
*  Calculate unknown 3-D vertex using a least squares fitting procedure.
*
*  Input Parameters:
*  ntrk            integer variable
*                  # of tracks to fit (length of list_trk)
*
*  list_trk(*)     integer  array
*                  List of tracks in track list to fit (length = ntrk)
*
*  update_tracks   integer variable
*                  0 ==> don't update input track parameters
*                  1 ==> update input track parameters
*                  2 ==> update input track parameters & covariance matrices
*                        and replace the original unconstrainted parameters
*                        by the new ones. ****** This is a dangerous option.
*
*  luse_vtx        logical variable
*                  TRUE  ==> use the value of vtx(1-3) as the starting value
*                  FALSE ==> compute starting value with straight line approx.
*
*  vtx(3)          double precision array
*                  Initial values of (x,y,z)
*
*  Output Parameters:
*  vtx(3)          double precision array
*                  Updated values of (x,y,z)
*
*  Vvtx(3,3)       double precision array
*                  3x3 covariance matrix of vtx(1-3)
*
*  chisq           double precision variable
*                  Chisquare of fit
*
*  error           integer variable
*                   0  if all OK
*                  >0  if error
*
*  Other routines:
*
*  Notes:
*
*  Author:   Paul R Avery      Created:  Mon Jul 28 07:05:06 EDT 1997
*
*  Major revisions:
*
* end_doc