code: def _set_tag(self, tag=None, tags=None, value=True):
          
          existing_tags = self._requirements.get("tags")
          if tags and not tag:
              existing_tags = merge(existing_tags, tags)
              self._requirements["tags"] = existing_tags
          elif tag and not tags:
              existing_tags[tag] = value
              self._requirements["tags"] = existing_tags
docs: Sets the value of a specific tag or merges existing tags with a dict of new tags.
          Either tag or tags must be None.
  
          :param tag: Tag which needs to be set.
          :param tags: Set of tags which needs to be merged with existing tags.
          :param value: Value to set for net tag named by :param tag.
          :return: Nothing