PHP Solr部分更新不可能吗？

I am trying to figure out whether it is possible to do a partial update on some solr documents but could not find any useful solution yet. Tried the following:

$client->setServlet(SolrClient::SEARCH_SERVLET_TYPE, "update");
$doc = new SolrInputDocument();
$doc->addField('keywords', '{set:'.$tag.'}');
$updateResponse = $client->addDocument($doc);
$updateResponse->getResponse()

Response:

SolrObject Object
(
[responseHeader] => SolrObject Object
    (
        [status] => 0
        [QTime] => 2
    )
)

Search output:

{
    "url": "http://de.wikipedia.org/wiki/Bier",
    "keywords": [
      "{set:bier}",
    ],
    "_version_": 1458395909033296000
  }

Keywords is a multivalued field. Obviously, $doc->addfield() converts my solr directive into a string, which is logical. So is there any method which allows me to pass 'set', 'add' or 'inc' to update specific fields?

thanks in advance!