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cd `brew --prefix`
rm -rf Cellar
brew prune
rm -rf Library .git .gitignore bin/brew README.md share/man/man1/brew
rm -rf ~/Library/Caches/Homebrew

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This should also revert your /usr/local folder to its pre-Homebrew days. See the Homebrew installation wiki (http://github.com/mxcl/homebrew/wiki/installation) for more information.