brewkit.rb changes ENV destructively, so lets not do that everytime a formula
is required. Now it's possible for other tools to require a formula
description without worrying about side-effects.
Guile is a library designed to help programmers create flexible applications. Using Guile
in an application allows programmers to write plug-ins, or modules (there are many names,
but the concept is essentially the same) and users to use them to have an application fit
their needs.