private function checkEntityArray( array $entityArray, &$error = null )
    {
        // get class
        if( $existing = $this->classGet() ) {
            foreach( $entityArray as $value ) {
                if( !( $value instanceof $existing ) ) {
                    $error = "Can't add entity of class `".get_class( $value )."` to container of class `{$existing}`.";
                    return false;
                }
            }
        }
        $error = '';
        return true;
    }