The nonexistent operator (along = v. == lines)

[Chris Dollin]

Dave Eisen writes (apropos of the ``xor'' discussion):

   #define XOR(a,b)	(!!(a) ^ !!(b))
   does what you want.

Hmm ... we can strip one of the !'s from each pair to get

    (!(a) ^ !(b))

If the XOR is likely to be used in conditional contexts (rather than value
contexts) then I'd guess that more compilers could optimise equality tests than
^ instructions (because more machines have some sort of branch-if-(n)eq), so
how about

#define XOR(a,b)   (!(a) != !(b))


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