subroutine kfit_3momentum(ktrk, update_track, p3c, chisq, error)
*
* begin_doc
*
*  Fit a track by constraining it to a fixed 3-momentum.
*
*  Note: If the track is not updated here, you can update it later by
*  calling kfit_update_tracks.
*
*  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
*
*  p3c(4)        double precision variable
*                3-momentum to constrain to
*
*  Output Parameters:
*  chisq         double precision variable
*                Chisquare of fit
*
*  error         integer variable
*                 0 ==> All OK
*                >0 ==> Error
*
*  Other routines:
*
*  Notes:
*
*  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
*   The energy constraint is imposed by the method of Lagrange multipliers
*
*   chisq = (alpha-alpha0)(t) * Valpha0(inv) * (alpha-alpha0)
*             + 2*lambda(t)*(D*dalpha + d)
*
*   where
*    alpha0 are the unconstrained parameters,
*    alpha are the final parameters and
*    D*dalpha + d = 0 is the linearized constraint equation.
*
*  The solution is
*
*       alpha = alpha0 - Valpha0*D(t)*lambda
*      lambda = VD * (D*dalpha0 + d)
*          VD = [D * Valpha0 * D(t)](inv)
*      Valpha = Valpha0 - Valpha0*D(t)*VD*D*Valpha0
*
*   The 3-momentum constraint has the form
*
*        Px - Pxc = 0
*        Py - Pyc = 0
*        Pz - Pzc = 0
*
*   So D and d can be written (expanding about P)
*
*           | 1  0  0 |
*       D = | 0  1  0 |
*           | 0  0  1 |
*
*           | Px1 - Pxc|
*       d1= | Py1 - Pyc|
*           | Pz1 - Pzc|
*
*   The covariance matrix of the resulting track can be computed as
*
*       Valpha = Valpha0 - Valpha0*D(t)*VD*D*Valpha0
*  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
*
*  Author:   Paul Avery      Created:  Fri Aug 29 16:12:52 EDT 1997
*
*  Major revisions:
*
*
* end_doc