docs: def addItemTag(self, item, tag):
          """
          Add a tag to an individal item.
  
          tag string must be in form "user/-/label/[tag]"
          """
          if self.inItemTagTransaction:
              # XXX: what if item's parent is not a feed?
              if not tag in self.addTagBacklog:
                  self.addTagBacklog[tag] = []                
              self.addTagBacklog[tag].append({'i': item.id, 's': item.parent.id})
              return "OK"
          else:
              return self._modifyItemTag(item.id, 'a', tag)