Book is updated

I'm updating the book, quite some time has passed since the last update, and only two small changes have been made in the meantime.

Changes since the last update:

• simplified description of Wave movement, if activated by Serpent
• removed Terms from Prerequisites

The latest update to the book was made on September 9, 2022, and the version is 20220908.223634.

I also removed all links to book preview. 

Book is close to finished, I don't plan any major changes, and certainly not stretched over many months, or years. Some smaller, but still significant changes could make it into the book, e.g. extended Serpent movement. Again, this is not something I'll be doing in a hurry.

For now, I'll try to work more on library.

Edit: just as I wrote "work more on library", it occurred to me that Wave should be made transparent, i.e. it makes sense to allow other pieces to pass-through Wave, just as Wave can pass-through all the other pieces. This would also make defending Wave easier, since it has to stretch out to activate anything, and most interesting pieces to activate are opponent's.

Since the book is the most important part of the project, changes will be made immediately, after which coding will resume as usual.