+ installed. (Probably you don't. If you don't know what I'm talking about, you don't have these.)
+
+ If you don't have these, but want to try this route, you can read about
+ the MacPorts system at <http://www.macports.org/>.
+ This automates the building of Unix-type software on your Mac; it
+ makes it a lot easier to check for dependencies, use more-recent
+ versions of things, and so on. (Though as it happens, MacPorts only has an older version of
+ our chosen implementation of Scheme.)