puke.rb

  def subclasses_of(*superclasses) #:nodoc:
    subclasses = []

    superclasses.each do |sup|
      ObjectSpace.each_object(class << sup; self; end) do |k|
        if k != sup && (k.name.blank? || eval("defined?(::#{k}) && ::#{k}.object_id == k.object_id"))
          subclasses << k
        end
      end
    end

    subclasses
  end