Involutary cipher (like ROT13) for an alphabet with an odd number of symbols

ROT13 has the nice property of being an involution for an A-Z alphabet (26 letters), th开发者_如何学Cat is to say:

ROT13(ROT13(string-A-to-Z)) = string-A-to-Z ;

What is a simple cipher function for an alphabet with an odd number of symbols that has the same property? Obviously, a straight-up substitution won't work, but I'm looking for something nearly as simple.

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Either exclude one of the symbols in the alphabet from the cipher, or supplement it with a symbol not in the alphabet.

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Such a cipher is necessarily the product of disjoint transpositions, so if you had an odd number of letters, one would have to remain in place.

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You could "reverse" the alphabet, switching A with Z and B with Y etc.