grammar - a confusion about answer "yes" and "no" to some complicated question

I got confused about the usage of "yes" and "no".

I knew in English,"yes" or "no" should response to the meaning of sentence.

take a example. assuming a stuff is available.

     is it available? -yes.
     isn't it available? -yes.

but I not sure to say "yes" or "no" when someone ask a complicated question with double-negatives.

     is it unavailable? 
     isn't it unavailable?

Answer

Is it unavailable?

(a) If it is unavailable then you answer:

"Yes." or "Yes, it is unavailable."

(b) If, on the other hand, it is not unavailable then it is available. You have a choice and can answer:

"No." or "No it isn't unavailable.

or you can say:

"No, it is available." or "No. In fact it is available."

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Discussion

So far I have only dealt with the first part, i.e. the question "Is it unavailable?" In theory I should now go on to explain the second part, i.e. "Isn't it unavailable?"

However

In real-life, no-one would ever ask that.

If you try to memorise all these different responses you will become hopelessly confused. The key is not memorisation, it is understanding the purpose of the question.

A question is a request for information. The best way to deal with it is simply to give the information! You know whether X is available or not. Therefore you can simply say which is true. No matter how complicated the question, reply by talking about availabilty.

Example

Me: Good morning, I want some X. Is true or not true that X is available or isn't available, or is it?

You: We have plenty of X available. How much would you like?

or

You: I'm sorry, we don't have any X available today. Would you like me to order some for you?

Conclusion

In real life we don't try to imitate the form of a complicated question with double-negatives -- we simply answer.