anchor.diff:         external is_integer
   c
          pname = "lattice_read_file"
  -       write(*,*) "reading lattice from file"
          lat = 0.0d0
          ifn = 13
          inquire(file=fname,exist=ofile)
  
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:          write(*,*) (lat(i,1),i=1,3)
           write(*,*) (lat(i,2),i=1,3)
           write(*,*) (lat(i,3),i=1,3)
           if(atom1_id.ne.0) then
             write(*,*) "1st atom ID: ",atom1_id
           else
             write(*,*) "1st atom tag: ",atom1_tag
           end if
           if(atom2_id.ne.0) then
             write(*,*) "2nd atom ID: ",atom2_id
           else
             write(*,*) "2nd atom tag: ",atom2_tag
           end if
           write(*,*) "Number of bins:",nb
           write(*,*) "Maximum radius:",rmax
           write(*,*) "Input file(s)"
           do i=1,nfil-1
             write(*,*) trim(infile(i))
           end do
           write(*,*) "Output file:"
           write(*,*) trim(infile(nfil))
         end if
  
         allocate(gr(nb))
  