subroutine kvir_two_vertex(kQ, num_neut, list_neut,
     *                           update_tracks, z1z2, z10, Vz10,
     *                           chisq, kH, error)
*
* begin_doc
*
*  Build a particle H which emerges from a primary vertex (P) and
*  decays at a secondary vertex (S) into a daughter Q that has
*  position information plus neutrals that do not.
*
*                 H --> Q + N1 + N2 + ...
*
*  Particle H is placed at its production vertex P.
*
*  The constraints are (1) particle H emerges from an existing vertex
*  P (having a known covariance matrix) and (2)  daughter Q intersects
*  H at a location S determined from the fit (3 unknowns). The number
*  of degrees of freedom is 2*2 - 3 = 1.
*
*  Input Parameters:
*  kQ             integer variable
*                 Track # of daughter track Q (has position information)
*
*  num_neut       integer variable
*                 Number of neutral tracks
*
*  list_neut(*)   integer array
*                 List of neutral tracks
*
*  update_tracks  integer variable
*                   0 ==> Calculate chisquare (do not update input tracks)
*                   1 ==> Calculate chisquare & update input tracks
*
*  z1z2(6)        double precision array
*                 Initial guess of double vertex (it is not updated)
*
*  z10(3)         double precision array
*                 (x,y,z) of prior primary vertex (it is not updated)
*
*  Vz10(3,3)      double precision array
*                 3x3 initial covariance matrix of z10
*
*  kH             integer variable
*                 Track number to put resulting virtual particle H
*
*  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 Nov 10 22:06:36 EST 1998
*
*  Major revisions:
*
*
* end_doc