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.
Tree is a recursive directory listing program that produces a depth indented
listing of files, which is colorized ala dircolors if the LS_COLORS
environment variable is set and output is to tty.
Max Howell
Adam Vandenberg
Brett Porter