SUBROUTINE KWTOWZ(TRACK1, ZS, TRACK2, LERROR)
 
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
C  Takes a helix expressed in W form and calculates the new parameters
C  at the position z = ZS.
C
C  TRACK1   real array (read)
C           Initial track parameters (Px, Py, Pz, E, x0, y0, z0, pt, p, Q)
C
C  ZS       real variable (read)
C           Z position to which the track should be projected
C
C *TRACK2   real array (write)
C           Track parameters at z = ZS
C
C *LERROR   logical variable (write)
C           TRUE if error projecting to this z position
C  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>