fcn caesarCodec(str,n,encode=True){
   var [const] letters=["a".."z"].chain(["A".."Z"]).pump(String);  // static
   if(not encode) n=26 - n;
   m,sz := n + 26, 26 - n;
   ltrs:=String(letters[n,sz],letters[0,n],letters[m,sz],letters[26,n]);
   str.translate(letters,ltrs)
}