\ Ensure the output char is in the correct range:
: modulate \ char base -- char
  tuck n:- 26 n:+ 26 n:mod n:+ ;

\ Symmetric Caesar cipher.  Input is text and number of characters to advance
\ (or retreat, if negative).  That value should be in the range 1..26
: caesar \ intext key -- outext
  >r
  (
    \ Ignore anything below '.' as punctuation:
    dup '. n:> if
      \ Do the conversion
      dup r@ n:+ swap
      \ Wrap appropriately
      'A 'Z between if 'A else 'a then modulate
    then
  ) s:map rdrop ;

"The five boxing wizards jump quickly!"
dup . cr
1  caesar dup . cr
-1 caesar . cr
bye