{{
  : rotate  ( cb-c ) tuck - 13 + 26 mod + ;
  : rotate? (  c-c )
    dup 'a 'z within [ 'a rotate ] ifTrue
    dup 'A 'Z within [ 'A rotate ] ifTrue ;
---reveal---
  : rot13   (  s-s ) dup [ [ @ rotate? ] sip ! ] ^types'STRING each@ ;
}}

"abcdef123GHIJKL" rot13 dup puts cr rot13 puts
"abjurer NOWHERE" rot13 puts