(@racket[wire-value-cache] is just a performance enhancement, so that wire values don't have to be computed multiple times.)
One gotcha when using syntax transformers is that newly defined identifiers can mask others. For instance, one of the wires in our input is named @tt{if}. When our syntax transformer defines the @tt{if} function, it will override the usual meaning of @racket[if]. There are plenty of elegant ways to handle these kind of namespace collisions. But because this is a puzzle, we'll take the cheap way out: we won't use @racket[if] elsewhere in our code, and instead use @racket[cond].
One gotcha when using syntax transformers is that newly defined identifiers can mask others. For instance, one of the wires in our input is named @tt{if}. When our syntax transformer defines the @tt{if} function, it will override the usual meaning of @racket[if]. There are plenty of elegant ways to prevent these name collisions. (The most important of which is called @italic{syntax hygiene}, and permeates the design of Racket's syntax-transformation system.) But because this is a puzzle, we'll take the cheap way out: we won't use @racket[if] elsewhere in our code, and instead use @racket[cond].
