public static function _check_partial_fit_first_call(Classifier $clf, \CArray $classes = null)
    {
        if($clf->classes_() == null && !isset($classes) ) {
            throw new ValueErrorException("classes must be passed on the first call to partial_fit.");
        }

        if(isset($classes)) {
            if($clf->classes_()) {

            } else {
                $clf->classes_(CArray::unique($classes));
                return true;
            }
        }
        # classes is None and clf.classes_ has already previously been set:
        # nothing to do
        return false;
    }