Buy-Url Variable Declaration in TFL Example

Hey there. Good to see this forum around. I’m placing this question under typography and fonts as it seems the best fit. I was going through Butterick’s pollen/racket example for TFL and noticed that on the index page there is a declaration of the purchase URL for the book that looks like this: ◊home-link[buy-url]{get the book}

I understand that [buy-url] is a variable that must be defined somewhere, but I cannot for the life of me figure out where. I don’t see it in the pollen.rkt file. Has anyone else seen this variable and where it is defined?

Edit: Changed to Computation after seeing description.

buy-url is defined in scribblings/pollen-rkt.scrbl