At first I thought of explaining the tracer in the manner of a teacher, starting with theoretical motivations and simple examples, illustrated helpfully. Realizing in time that as I anticipated no readers, or perhaps only a single ideal reader, and in either case the effort at pedagogical soundness would be wasted, I begin with two complete modules of code: neither the first written nor the first imported at runtime; yet if I were to explain this code adequately (which I shall not) the whole of the tracer would be made lucid.