With mild horror, I realize ¡­

With mild horror, I realize that I failed to introduce the data structure mentioned in the second part (9d) of the current exercise, so-called property lists. In other words, the second occurence of (d) (starting with Use a property list ...) should not have been there. Please just ignore the duplicate part (9d), i.e. pretend it was not there! Furthermore, part (10d) somewhat mysteriously ends with the suggestion (see below). This is a copy-and-paste error, and there is nothing more to see below; so please just ignore that last phrase in part (10d). My apologies for the confusion!