Ruslan Sharipov wrote a text on the subject : http://arxiv.org/pdf/math/0405323v1.pdf http://arxiv.org/pdf/math/0405323v1.pdf It is very good and explains all

Ruslan Sharipov wrote a text on the subject : http://arxiv.org/pdf/math/0405323v1.pdf http://arxiv.org/pdf/math/0405323v1.pdf It is very good and explains all the proofs and definitions very well.