The final draft was edited by the three initiators, with the aim of improving the organization of the manuscript by reordering contributions and comments, reducing, where possible, repetition and ensuring a certain level of uniformity in notation and clarity of presentation. However, to retain the spontaneity of the discussion and reflect the multitude of views presented, reorganization was kept to a minimum. As a consequence, some themes may be revisited in different contexts throughout the paper -much as would happen in a lively round table discussion.