anchor.diff:          if (flag == 0) then
              flag = 2
           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 "-f": missing argument'
              stop
           end if
        case('-d', '--digits')
  
positive.code:         iarg = iarg + 1
          call read_opt_arg(iarg,nargs,n_digits,err)
          if (err == 2) then
             write(0,*) 'ERROR ! missing argument: '//trim(arg)//' <n>'
             stop
          end if
       case default
          write(0,*) 'ERROR ! unkown argument '//trim(arg)
          stop
       end select
    end do args_loop
  
negative.code:   use arguments, only: read_opt, read_opt_arg
    use parser, only: parser_nfields,remove_comments, parse_next_line
    implicit none
  
    character(len=string_size) :: data_type
    integer                    :: nvars=0, nclasses=0
    integer,       allocatable :: seqs(:,:)
  