REPEAT
        INPUT A$
        PRINT FNrot13(A$)
      UNTIL FALSE
      END

      DEF FNrot13(A$)
      LOCAL A%,B$,C$
      IF A$="" THEN =""
      FOR A%=1 TO LEN A$
        C$=MID$(A$,A%,1)
        IF C$<"A" OR (C$>"Z" AND C$<"a") OR C$>"z" THEN
          B$=B$+C$
        ELSE
          IF (ASC(C$) AND &DF)<ASC("N") THEN
            B$=B$+CHR$(ASC(C$)+13)
          ELSE
            B$=B$+CHR$(ASC(C$)-13)
          ENDIF
        ENDIF
      NEXT A%
      =B$