# File src/PlugMan.rb, line 395
    def PlugMan.required_by(plugin_name)
      ret = []
      # iterate all the plugins that declare this plugin as required
      @registered_plugins.each do |nm, ob|
        ret << nm if ob.requires.include?(plugin_name)  || (ob.extends && ob.extends.keys.include?(plugin_name))
      end
      ret.uniq
    end