subroutine swap(np,wtp)
c---swap coordinates of particle np with particle ngood
c----------------------------------------------------------------------
      save
c
      include 'param_sz.h'
      include 'constcom.h'
      include 'coordcom.h'
      include 'ncordscom.h'
      include 'pcordcom.h'
      include 'syscom.h'
      include 'ucom.h'
c
      common/aswap/acor(7,imaa)	
      common/back/bmax,byz0,byzs,pzowmin,pzowmax,iback 
      common/jones/ajones,zjones,zcath(imaa)
      common/part/nparticle(imaa)
      common/pnum/ipnum(imaa)
      dimension xyz(7)
c--------------------------------------------------------------------------
c*
      xyz(1)=x
      xyz(2)=bgx
      xyz(3)=y
      xyz(4)=bgy
      xyz(5)=z
      xyz(6)=bgz
      xyz(7)=rne
c
c     for back bombardment
      if(iback.eq.1) then
           if(xyz(7).le.1.and.xyz(6).le.0.) then
           write(nback1) xyz(1),xyz(2),xyz(3),xyz(4),xyz(5),xyz(6),
     1                     erest*(gamma-1.),phizero(np),wtp
           endif
      endif 
      if(np.eq.ngood)return
c      real t
      do 10 i=1,6
      cord(i,np)=cord(i,ngood)
      cord(i,ngood)=xyz(i)
   10 continue
      do 20 j=1,7 ! alpha magnet
      atmp=acor(j,np)
      acor(j,np)=acor(j,ngood)
      acor(j,ngood)=atmp
   20 continue
      cord(7,np)=cord(7,ngood)
      cord(7,ngood)=xyz(7)
      wt=weight(np)
      weight(np)=weight(ngood)
      weight(ngood)=wt
      ph=phizero(np)
      phizero(np)=phizero(ngood)
      phizero(ngood)=ph
      iparticle=nparticle(np)
      nparticle(np)=nparticle(ngood)
      nparticle(ngood)=iparticle
      gam(np)=gam(ngood)
      gam(ngood)=gamma
      it=ipnum(np)
      ipnum(np)=ipnum(ngood)
      ipnum(ngood)=it
      t=zcath(np)
      zcath(np)=zcath(ngood)
      zcath(ngood)=t
      return
      end
c++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++*