subroutine sextupole(zn)
c-----------------------------------------------------------------------
c
      include 'param_sz.h'
      include 'constcom.h'
      include 'pcordcom.h'
      include 'syscom.h'
c
c-----------------------------------------------------------------------
      ne=rne
      sl=zn-z
      xp=bgx/bgz
      yp=bgy/bgz
      con=1.+xp**2+yp**2
      z=zn
      caysq=el(4,ne)/(brhof*bgz)
      caysqtsq=caysq*sl**2
      xn=x+sl*xp-caysqtsq*(.5*x**2+x*xp*sl/3.+(xp*sl)**2/12.
     &                   -(.5*y**2+y*yp*sl/3.+(yp*sl)**2/12.))
      xpn=xp-caysq*sl*(x**2+x*xp*sl+xp**2*sl**2/3.
     &               -(y**2+y*yp*sl+yp**2*sl**2/3.))
      yn=y+sl*yp+caysqtsq*(x*y+x*yp*sl/3.+xp*y*sl+xp*yp*sl**2/6.)
      ypn=yp+caysq*sl*(2.*x*y+(x*yp+xp*y)*sl+2.*xp*yp*sl**2/3.)
      x=xn
      y=yn
      bgx=xpn*bgz
      bgy=ypn*bgz
      bgz=bgz*sqrt(con/(1.+xp**2+yp**2))
      return
      end
c++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++*