anchor.diff:          ! prm file
           iarg = iarg + 1
           call read_opt_arg(iarg,nargs,rst_file,err)
  -        if (err == 1) then
  +        if (err == 2) then
              write(0,*) 'ERROR ! missing argument: '//trim(arg)//' <filename>'
  +           stop
  +        end if
  +     case('-d', '--digits')
  +        ! the number of digits to the right of the decimal point
  +        ! default: print with full precision
  +        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
  
positive.code:         stop
       end select
    end do args_loop
  
    call units_open_unf(rst_file,'old',unt,err)
    
  
negative.code:   character(len=long_string_size) :: syntax = 'syntax'
    character(len=long_string_size) :: rst_file
    integer :: iv,jv,is,js,k
    integer :: n_digits = -1
    character(len=1) :: protein_alphabet(21) = &
         ['A','C','D','E','F','G','H','I','K',&
         'L','M','N','P','Q','R','S','T','V','W','Y','-']
    character(len=1) :: nuc_acid_alphabet(6) = &
         ['A','C','G','T','U','-']
    character(len=long_string_size) :: frmt
    
    ! read arguments
    call get_command(cmd)
  