Bí vs is construction
After reading Wikipedia (http://en.wikipedia.org/wiki/Irish_syntax#Answering_questions_with_the_copula), I'm a bit confused about using bí and is in Irish. If you use bí to describe the subject with an adjective, and is to equate a subject with something else, then how come "Tá Seán ina dochtúir." is the correct way to say "Sean is a doctor."? Why not use is?
Wikipedia says: Is dochtúir é Seán sounds more permanent—it represents something absolute about Seán; it is a permanent characteristic of Seán that he is a doctor. This is known as an individual-level predicate. In the sentence Tá Seán ina dhochtúir, one says rather that Seán performs the job of a doctor, that he is a doctor at the moment, that he has become a doctor. This is known as a stage-level predicate.
My understanding of this: If me and Seán were into roleplaying, and Seán wanted to play Gandalf... you could say he's a wizard with tá, because you're not equating the subject to something else, a wizard in this case; he isn't really a wizard, it's just some temporary attribute he's taken on for this occasion.