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Generate the answer for a given question. The question requires common knowledge of English. Make sure that the answer is in detail, long, consistent with the context, and requires general knowledge of English. To be less repetitive, please vary your language for each question. Give a proper explanation to answer, and if necessary, provide the definition of various terms. And conclude your answer if it's long. If you have used other resources (e.g., Merriam Webster's.) to answer the question, include citations in your answer. You don't need to paraphrase the context of your resources, but make sure that your answer is consistent and complete.

Possible Duplicate: What rules make 'Remember me, who am your friend' grammatical? This is a line spoken by the Emperor to Luke in Star Wars. I always wondered if this is grammatically correct. Luke says something like 'You are mistaken ...' which the Emperor answers with No, it is you who are mistaken! Why wouldn't he say No, it is you who is mistaken! instead? I don't know what rules apply here but my stomach tells me the latter is (at least also) correct, although you would say 'you are mistaken'. It feels as if he should be referring to 'you' in the third person. Could you please shed some light? Are both correct or if not which one is correct? And why?.
Output:
The sentence It is you who are mistaken is a Cleft sentence, derived from the base sentence (shown here with focussed subject You) You are mistaken by the Clefting process, which extracts the focussed NP (you) to be the predicate of a dummy clause with It subject and some form of be as verb (generating It is you in this case), and then making the non-focussed rest of the original sentence into a relative clause modifying the focus NP (generating who are mistaken in this case). Verb agreement is invariant under clefting, so if the predicate is are mistaken in the original, it will still be are mistaken in the clefted variant.