Exercise 4.52 - if-fail that lets you 'catch' the failures and return a custom value
tried TDD-style refactoring, but just wound up writing a few rationalizing comments
Exercise 4.53 - if-fail + permanent-set! to explore all alternatives and compile multiverse's results in a single data structure. this subsection's exercises have been uncharacteristically easy... a second edition effect?
Exercise 4.54 - implementing (require) as a special form instead of a procedure in terms of (amb)
passing comment