public function addTag($id, $tag)
{
$tag = $this->tagPrefix . $tag;
if ($this->getTagAdapter()->contains($tag)) {
$identifiers = $this->getTagAdapter()->get($tag);
} else {
$identifiers = [];
}
// array may contain not unique values, but I can't see problem here
$identifiers[] = $this->prepareId($id);
return $this->getTagAdapter()->set($tag, $identifiers);
}