public function setAuthUserName( ?string $user ) : MySQL
   {

      if ( ! $this->_supportedAttributes->get( 'user' )->validateValue( $user ) )
      {
         throw new ArgumentException(
            'user', \str_repeat( '*', min( 64, \mb_strlen( $user, 'utf-8' ) ) ), 'Invalid auth user!' );
      }

      $this->_attributes[ 'user' ] = $user;

      return $this;

   }