subroutine buncher(phase,iend,ibeg)
c---transformation for zero-length buncher
      save
c
      include 'param_sz.h'
      include 'constcom.h'
      include 'pcordcom.h'
      include 'syscom.h'
      include 'ucom.h'
c--------------------------------------------------------------------------
c*
      ne=rne
      cay=el(8,ne)
      rsq=x*x+y*y
      dwm=el(4,ne)
      csrs=cay*cay*rsq
      bgzin=bgz
      dg=dwm*xi0(csrs)*sin(phase)/erest*sign(1.,bgzin)
      g=sqrt(1.+bgz**2)+dg
c-----allow particle to go backward
      if (g.lt.1.) then
      g=1+(1-g)
      bgz=-sign(sqrt(g**2-1.),bgzin)
      else
      bgz=sign(sqrt(g**2-1.),bgzin)
      endif
      bgzb=.5*(bgzin+bgz)
      dbgr=dwm*cos(phase)*cay*xi1ox(csrs)/(bgzb*erest)
      bgx=bgx+dbgr*x
      bgy=bgy+dbgr*y
      gamma=sqrt(1.+bgx**2+bgy**2+bgz**2)
      if(bgz.lt.0.)then
      ibeg=1
      else
      iend=1
      endif
      return
c   10 bgz=0
c      return
      end
c++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++*