[ $ "" swap
  witheach
    [ dup  char A char M 1+ within
      over char a char m 1+ within or
      iff [ 13 + ]
      else
        [ dup  char N char Z 1+ within
          over char n char z 1+ within or
          if [ 13 - ] ]
      join ] ]                            is rot-13 ( $ --> $ )