Specifically, for NaN (and for definitions of NULL that act similarly), it breaks pretty much all useful math due to violating the axiom that `x = x`. All the o

Specifically, for NaN (and for definitions of NULL that act similarly), it breaks pretty much all useful math due to violating the axiom that `x = x`. All the other weird things it does are totally fine, but an absorbing element still isn't allowed to do that.