    # File lib/facets/core/enumerable/commonality.rb, line 22
      def commonality( &block )
        had_no_block = !block
        block ||= lambda { |item| item }
        result = Hash.new { |hash, key| hash[key] = Array.new }
        self.each do |item|
          key = block.call(item)
          result[key] << item
        end
        result.reject! do |key, values|
          values.size <= 1
        end
        #return had_no_block ? result.values.flatten : result
        return result
      end