subroutine kvtx_two_vertex_life(kQ, num_neut, list_neut,
     *                                update_tracks, luse_z,
     *                                z1z2, Vz1z2, z10, Vz10,
     *                                ctau, sig_ctau, chisq, error)
*
* begin_doc
*
*  Determine two vertex locations and the proper lifetime of primary
*  particle H in the process
*
*                 H --> Q + N1 + N2 + ...
*
*  where Q has position information and the neutral N's do not. Let H
*  emerge from a vertex z1 (having a prior covariance matrix) and decay
*  at the unknown vertex z2. We want to determine the following 7
*  quantities:
*
*      z1(1-3)    Primary vertex
*      z2(1-3)    Secondary vertex
*      ctau_H     c*tau of particle H in going from z1 to z2
*
*  There are 6 constraints:
*
*    (1-3) H intersects primary vertex at z1
*    (4-6) Q intersects H at z2.
*
*  The vertex z1 is updated by the fit while 5 unknown parameters have
*  to be determined: z2(1-3), ctau_H and the arc length s_Q from z2 to
*  particle Q. The number of degrees of freedom is therefore 2*3 - 5 = 1.
*
*  Input Parameters:
*  kQ              integer variable
*                  Track number of particle with position information
*
*  num_neut        integer variable
*                  Number of neutral tracks
*
*  list_neut(*)    integer  array
*                  List of neutrals
*
*  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 z1z2(1-6) as starting vertices
*                  FALSE ==> use z10(1-3) as primary, determine secondary
*
*  z1z2(6)         double precision array
*                  Initial values of z1(1-3), z2(1-3)
*
*  z10(3)          double precision array
*                  Prior primary vertex (z1)
*
*  Vz10(3)         double precision array
*                  3x3 covariance matrix of z10
*
*  Output Parameters:
*  z1z2(6)         double precision array
*                  Final values of z1(1-3), z2(1-3)
*
*  Vz1z2(6,6)      double precision array
*                  Covariance matrix of z1z2
*
*  ctau            double precision variable
*                  c*tau of primary particle H
*
*  sig_ctau        double precision variable
*                  Error on ctau
*
*  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:  Sat Nov 07 07:05:06 EST 1998
*
*  Major revisions:
*
* end_doc