Ok and how can I change head to accept an array. If I pass an array as an argument for head I get:
```
Error: can't convert Array into String
Please report this bug:
    https://github.com/mxcl/homebrew/wiki/troubleshooting
/usr/local/Library/Homebrew/dependency_collector.rb:179:in `extname'
/usr/local/Library/Homebrew/dependency_collector.rb:179:in `parse_url_spec'
/usr/local/Library/Homebrew/dependency_collector.rb:159:in `resource_dep'
/usr/local/Library/Homebrew/dependency_collector.rb:61:in `parse_spec'
/usr/local/Library/Homebrew/dependency_collector.rb:47:in `build'
/usr/local/Library/Homebrew/dependency_collector.rb:32:in `add'
/usr/local/Library/Homebrew/software_spec.rb:40:in `url'
/usr/local/Library/Homebrew/formula.rb:725:in `head'
/usr/local/Library/Formula/xulrunner.rb:44
/usr/local/Library/Homebrew/formulary.rb:47:in `require'
/usr/local/Library/Homebrew/formulary.rb:47:in `klass'
/usr/local/Library/Homebrew/formulary.rb:97:in `get_formula'
/usr/local/Library/Homebrew/formulary.rb:182:in `factory'
/usr/local/Library/Homebrew/formula.rb:443:in `factory'
/usr/local/Library/Homebrew/extend/ARGV.rb:12:in `formulae'
/usr/local/Library/Homebrew/extend/ARGV.rb:12:in `map'
/usr/local/Library/Homebrew/extend/ARGV.rb:12:in `formulae'
/usr/local/Library/Homebrew/cmd/install.rb:43:in `install'
/usr/local/Library/brew.rb:91:in `send'
/usr/local/Library/brew.rb:91
```