/**
  * get the top concepts for a vocabulary
  *
  * @return void
  * @param  Vocabulary $vocabulary
  */
 function getTopConcepts($vocabulary, $ts = null)
 {
     //get top concepts for vocabulary
     $c = new Criteria();
     $c->add(ConceptPeer::IS_TOP_CONCEPT, 1);
     $this->topConcepts = $vocabulary->getConcepts($c);
     return;
 }