Thomas Seeling wrote in a message to Tom Torfs:

 TT>>          c = (((c-'a')+13)%26)+'a';
 TT> This assumes contiguous letters of course.

 TS> afair Ansi-/ISO-C forces letters and digits to be ordinally
 TS> contiguous. 

You remembered wrong. Only the digits have such a guarantee. For letters this
would not be feasible, since in the second most used family of character sets
(EBCDIC) this isn't the case.

2.2.1 Character sets (this is 5.2.1 in ISO numbering)



the 10 decimal digits 

         0  1  2  3  4  5  6  7  8  9



In both the source and execution basic character sets, the value of each
character after 0 in the above list of decimal digits shall be one greater
than the value of the previous.

greetings,
Tom
tomtorfs@village.uunet.be

--- timEd/2 1.10.y2k+