nan question

[Janice Caron]

Think of NaN as meaning "I don't know the answer".

With that understanding, when you compare two NaNs with ==, you are
asking "Is one thing I don't know the answer to equal to another thing
I don't know the answer to". The absolutely correct answer should
really be "I don't know, but probably not", but since that can't be
expressed in a bool, we go with the second best answer: no.