there isn't always a yes/no/mu answer

Quite right, Bryan, in terms of all questions. But what I meant to imply (sort of paraphrasing Hofstadter) was that you can answer "yes/no/mu" to any question meant to elicit a "yes/no" answer

"Were you at X's house on the night of the 5th November?"
"The 5th November this year, you mean?"
"Why, were you doing something on 5th November last year?"
"Does that have any bearing on the case?"

It's pretty easy to show that "yes" and "no" are about the least likely answers to any question, actually

Fisk