Here's a better Ruby version:

    module Enumerable
      def custom_flatten
        inject([]) do |result, element|
          if element.respond_to?(:custom_flatten)
            result.concat(element.custom_flatten)
          else
            result.push(element)
          end
        end
      end
    end

Works on all Enumerables (Array, Hash, Set, etc)

How is Ruby's method and different than Groovy or JavaScript? I think their all pretty neat. Goes to show that there is always more than one way to skin a list.