def __partial(source, data={})
      if source.kind_of?(Source)
        Undies::Template.new(source, data, @_undies_io)
      else
        @_undies_io.current.partial(source.to_s)
      end
    end
def __partial(source, data={})
      if source.kind_of?(Source)
        Undies::Template.new(source, data, @_undies_io)
      else
        @_undies_io.current.partial(source.to_s)
      end
    end
call this to render partial source embedded in a template
 partial source is rendered with its own scope/data but shares
 its parent template's output object