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I think you have misinterpreted the meaning of "not applicable" - it doesn't mean "don't know", it means "the question makes no sense in this context".
In this case, consider a refrigeration unit that achieves its goal by using a fashionable Dyson vortex to generate a whirlwind of frost which it then blasts at the to-be-refrigerated [0] contents thereby cooling it. Clearly such a fridge cannot be frost-free - the use of frost is an integral part of its workings - hence the answer is not simply "no", but "not applicable".
While you can extend boolean arithmetic to cope with three-valued logic to cater for "don't know", I don't think you can construct a similar extension to cater for "not applicable" - I'd think of it like genetics, in the sense that it is neither dominant (like SQL's NULL) nor recessive (like perl's "undef").
Hugo
[0] Latin would be "refrigerandi"; you have to respect a language that understands the need for passive future gerundives.
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