Idempotency

More from ll1-discuss, this humorous tidbit from Guy Steele in a rambling thread (arguably the same thread mentioned in my previous post, but with significant drift) about laws, correctness, language, programming, and legal interpretation:

When the Clinton impeachment mess was going on, when I read
in the news about the definition of "sexual relations" that
had been agreed upon by the lawyers, my mathematical training
led me to notice immediately that the definition was not
symmetric; that is, that under that definition it was possible
for A to have "sexual relations" with B while at the same time
B was not having "sexual relations" with A. And Clinton spotted
that bug and tried to exploit it, which struck me as hackishly
brilliant (though politically stupid and morally arguable).

When a mathematician encounters the definition of a new relation,
he should always immediately ask: (a) Is it symmetric? (b) Is it
transitive? (c) Is it reflexive? (In the case of sexual relations,
I would say (a) yes, (b) it depends on whether you're concerned about
disease transmission, for example, and (c) let's not go there.)
(In the case of a binary operator, you ask whether it is commutative,
associative, has an identity, has idempotent values, and so on.)