Walls and chambers

We discuss the proof in the "rational case", state Lemma 6.17, work out the details of Lemma 6.18 and conclude the proof for the rational case (following Macrì-Schmidt's notes). We say very briefly what one needs in the general case.

We discuss wall and chamber structure in the alpha-beta plane as in Section 6.4, (maybe with the aid of drawings as in Proposition 3.7 of Macrì-Schmidt's notes)