def __yield
      return if (source = @_undies_source_stack.pop).nil?
      if source.file?
        instance_eval(source.data, source.source, 1)
      else
        instance_eval(&source.data)
      end
    end
def __yield
      return if (source = @_undies_source_stack.pop).nil?
      if source.file?
        instance_eval(source.data, source.source, 1)
      else
        instance_eval(&source.data)
      end
    end
Source handling methods
 call this to render template source
 use this method in layouts to insert a layout's content source