Normally a preface will give a list of the names of friends who have taken the trouble to read drafts of the manuscript, but I have found myself spontaneously adopting a slightly different and, I believe, more rigorous course. In the final stages of writing, over the last two years or so, I have accepted offers to participate in workshops where I could attempt a dry run of my ideas. As a consequence the work has had considerable feedback, but a price of participation is that versions of parts of the work have been published or are being published
