Applied review 2

Taken into account all remarks, except the implications of

p.1 col 2. I found strange to denote extensions with an arrow: F_p ->
K_{\ell^n}, why not using the usual notation "K_{\ell^n}/F_p" or the
inclusion "K_{\ell^n} \supset F_p".

which would have been too ugly just a few lines below