code:

		/** untraining of a document.
		 To remove just one document from the references.
		 @see updateProbabilities()
		 @see untrain()
		 @return bool success
		 @param string document id, must be unique
		 */
		function untrain($doc_id) {
			$ref = $this->nbs->getReference($doc_id);
			$tokens = $this->_getTokens($ref['content']);

			while (list($token, $count) = each($tokens)) {
				$this->nbs->removeWord($token, $count, $ref['category_id']);
			}

			$this->nbs->removeReference($doc_id);

			return true;
		}