The "txtadv+world.rkt" is the initial implementation of the
text-adventure game in Racket. The file name foreshadows a split of
the program into two modules later.

The "Model" section of the code is described in the article. The "The
world" section is the verbose world description that we will improve
in the next step. Sections from "Game state" on implement the game
engine, and those parts will remain essentially unchanged as we
improve the language for describing worlds.