    # File lib/chef/run_list/versioned_recipe_list.rb, line 32
          def add_recipe(name, version=nil)
            if version && @versions.has_key?(name)
              unless Chef::Version.new(@versions[name]) == Chef::Version.new(version)
                raise Chef::Exceptions::CookbookVersionConflict, "Run list requires #{name} at versions #{@versions[name]} and #{version}"
              end
            end
            @versions[name] = version if version
            self << name unless self.include?(name)
          end