anchor.diff:         i = i+1
          call my_get_command_argument(i,buffer,l,istatus)
          if(istatus.ne.0) goto 18
  -c       write(*,*) "argument ",i,buffer
          if(buffer.eq."-lattice") then
  -c          write(*,*) "reading lattice"
             do k=1,3
               i = i+1
               call my_get_command_argument(i,buffer,l,istatus)
               if(istatus.ne.0) goto 18
  -c            write(*,*) "argument ",i,buffer,is_number(buffer)
               if(is_number(buffer)) then
                 read(buffer,*) f
                 latv(k:3) = f
  -c              write(*,*) "lat",k,latv(k)
               else
                 if(k.gt.1) then
                   i = i-1 
  
positive.code:            ok=n.ne.0
           end if
           if(.not.ok) exit
           write(*,*) "number of atoms", n
           oc1=.false.
           call mask_all(n,c,atag,oc1,atom1_id,atom1_tag)
           oc2=.false.
  
negative.code:          gr = gr + gr0
           nf = nf+1
           if(nfrm.gt.0.and.nf.ge.nfrm) then
              if(overb) 
       +      write(*,*) "exiting because exceeded number of frames",nf,nfrm
              exit
           end if
         end do
  