anchor.diff:  c      ---------------------------      
   c      start checks/balances
   c      ---------------------------      
  +c      files
  +       if(nfil.lt.2) then
  +         message = "please provide input/output files"
  +         goto 911
  +       end if
   c      atom tags
          if(atom1_id.eq.0.and.atom1_tag.eq." ") then
            message = "please provide central atom id or tag"
  
positive.code:          write(*,*) "Output file: ",file_out
         end if
  
  c      figure out format for trajectory file
         if(aformat.eq." ") then
          i=INDEX(file_in,".",.true.) 
          aformat=file_in(i+1:)
          write(*,*) "format is ",aformat
         end if
  
         if(aformat.eq."trj") then
           call trj_read_header(fn_in,nvm,nva,nu,nprec)
  
negative.code:           end if
            go to 16
         else 
                 write(*,*) "file buffer", buffer
            nfil = nfil+1
            infile(nfil) = buffer
            go to 16
         end if 
         
  c      ---------------------------      
  c      end of command line parsing
  c      ---------------------------      
  