<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _set_tag(self, tag=None, tags=None, value=True):
 """ 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 """

        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