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

[Doug Gwyn]

In article <1991Apr3.203748.3226 at neon.Stanford.EDU> dkeisen at Gang-of-Four.Stanford.EDU (Dave Eisen) writes:
>#define XOR(a,b)	(!!(a) ^ !!(b))

or even
#define XOR(a,b) (!(a)^!(b))
which is, I think, the shortest fully correct solution to this
rather peculiar "requirement".