SUBROUTINE KWTOPX(TRACKW, X, P4)
 
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
C    Takes a helix expressed in W form to the point nearest the
C    point X(1-3) and calculates the 4-vector there.
C
C    Note: this routine needs the magnetic field BFLDKN which is set
C          by calling KSBFLD.
C
C  TRACKW   real array (read)
C           W track parameters (Px, Py, Pz, E, X, Y, Z, pt, P, Q)
C
C  X        real array (read)
C           (x,y,z) of point
C
C *P4       real array (write)
C           Momentum 4-vector (Px, Py, Pz, E) returned to caller
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>