anchor.diff:            end select
             call read_arg(iarg,nargs,data_file,err)
             if (err == 1) then
  -             write(0,*) '# ERROR # missing argument: '//trim(arg)//' <filename>'
  +             write(0,*) 'ERROR ! missing argument: '//trim(arg)//' <filename>'
                error_code = 1
                return
             end if
  
positive.code:           ! rst file
            call read_arg(iarg,nargs,rst_file,err)
            if (err == 1) then
               write(0,*) '# ERROR # missing argument: '//trim(arg)//' <filename>'
               error_code = 1
               return
            end if
         case default
            write(0,*) '# ERROR # invalid option '//trim(arg)
            error_code = 1
            return
         end select
      end do args_loop
  
      if (prm_file /= "" .and. rst_file /= "") then
         write(0,*) '# ERROR # either a rst or a prm file'
         error_code = 1
         return
      end if
  
negative.code:           ! prm file
            call read_arg(iarg,nargs,prm_file,err)
            if (err == 1) then
               write(0,*) 'ERROR ! missing argument: '//trim(arg)//' <filename>'
               error_code = 1
               return
            end if
  