Adds a tag to a query.
Tags are strings that identify a query. A query may have any number of tags. Tags are used to mark a query so that alter hooks may decide if they wish to take action. Tags should be all lower-case and contain only letters, numbers, and underscore, and start with a letter. That is, they should follow the same rules as PHP identifiers in general.
$tag: The tag to add.
QueryAlterableInterface The called object.
Overrides QueryAlterableInterface::addTag
public function addTag($tag) {
$this->query
->addTag($tag);
return $this;
}