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

No. The only garanteed is: digits are contigous. So '0' through '9' can be
simple in/decremented. But letters not.

But practise thows tha independant of chasets and real computers it would go.
And if you don't mix numbers and letters (like 0x41 for 'A') and uppercase and 
lowercase it would be independant of ASCII or EBCDIC.

But the standard can not and will not guaratee that for all computers and each 
environment your program my run.

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