subroutine kvir_vertex_unknown(ntrk, list_trk, opt_trk,
     *                               update_tracks, luse_zv, zv,
     *                               chisq, kput, error)
*
* begin_doc
*
*  Build a new particle by adding the 4-momenta of several particles using
*  a vertex constraint. The initial vertex is unknown.
*
*  Input Parameters:
*  NTRK           integer variable
*                 # of input tracks
*
*  list_trk(*)    integer array
*                 List of KWFIT tracks.
*
*  opt_trk(*)     integer array
*                 opt_trk(it) = 0  => Track has no position info
*                 opt_trk(it) = 1  => Track is not used to determine vertex
*                                     but is constrained to pass through it
*                 opt_trk(it) = 2  => Track is used to determine vertex
*                 pizeros and photons probably should use opt_trk = 0
*
*  update_tracks   integer variable
*                   0 ==> Calculate chisquare (do not update input tracks)
*                   1 ==> Calculate chisquare & update input tracks
*
*  luse_zv        logical variable
*                  TRUE  ==> use zv(1-3) as the starting value
*                  FALSE ==> compute starting value with straight line approx.
*
*  zv(3)          double precision array
*                 Initial (x,y,z) vertex (it is not updated)
*
*  kput           integer variable
*                 Track number to put resulting virtual particle
*
*  Output Parameters:
*  chisq          double precision variable
*                 Chisquare of fit.
*
*  error          integer variable
*                 = 0  All OK
*                 > 0  Error
*
*  Other routines:
*
*  Notes:
*
*  Author:   Paul Avery      Created:  Wed Aug 20 01:06:36 EDT 1997
*
*  Major revisions:
*
*
* end_doc