anchor.diff:       select case(trim(arg))
        case('-h','--help')
           ! print help and exit
  -        write(0,*) 'help'
  +        write(0,100)
           stop
        case('-u')
           ! unformatted file
           if (flag == 0) then
              flag = 1
           else
  -           write(0,*) "either -u <unformatted_file > or -f <formatted_file> option"
  +           write(0,*) "ERROR: either -u <chk_file > or -f <formatted_file> option"
              stop
           end if
           iarg = iarg + 1
           call read_opt_arg(iarg,nargs,source,err)
  -        if (err == 2) then
  -           write(0,*) 'ERROR ! missing argument: '//trim(arg)//' <filename>'
  +        if (err > 0) then
  +           write(0,*) 'ERROR ! option "-u": missing argument'
              stop
           end if
        case('-f')
  
positive.code:      call units_open_unf(source,'unknown',unt,err)
       write(unt) nvars
       write(unt) nclasses
       write(unt) data_type     
       write(unt) ndata
       do id = 1,ndata
          write(unt) seqs(:,id)
       end do
       write(unt) prm
       close(unt)
       
    end select
  
  end program pchk
  
negative.code: ! Copyright (C) 2015-2017, Simone Marsili
  ! All rights reserved.
  ! License: BSD 3 clause
  
  