We are reading Mary Leng’s book in the Thursday Logic Seminar for the second part of term. It fell to me to introduce discussion of Chapters 5 and 6, and I found myself yesterday writing (very rapidly indeed) these brisk notes in trying to get my own thoughts clearer before the meeting. There was an excellent discussion which has led to a few after-the-event changes. But mostly these notes stay in the same rough-and-ready form, and I’m not going to have time to do better: so caveat lector! However, they might be of some interest to others intrigued by Mary’s project.