require 'Qt4'

module CattleLog
        class Gui < Qt::Widget
                include Singleton
                @main_window

                def initialize(parent = nil)
                        super

                        @main_window = Qt::MainWindow.new
                end
        end
end

./lib/gui.rb:5:in `const_missing': uninitialized constant CattleLog::Gui::Singleton (NameError)
        from ./lib/gui.rb:5
        from ./cattle_log.rb:6:in `require'
        from ./cattle_log.rb:6