private function assertKeysArrayValid($keys = null)
    {
        if ((!$keys) ||
            (!is_array($keys)) ||
            (!isset($keys['auth'])) || 
            (!is_array($keys['auth'])) ||
            (!isset($keys['auth']['req'])) || 
            (!is_array($keys['auth']['req'])) ||
            (!isset($keys['auth']['opt'])) || 
            (!is_array($keys['auth']['opt'])) ||
            (!isset($keys['args'])) || 
            (!is_array($keys['args'])) ||
            (!isset($keys['args']['req'])) || 
            (!is_array($keys['auth']['req'])) ||
            (!isset($keys['args']['opt'])) || 
            (!is_array($keys['auth']['opt']))
        ) {
            $msg = 'Invalid API keys provided. (HINT: Each '
                 . 'API class must define a $keys array with '
                 . 'specific key requirements. (HINT: View an /Api/[file] '
                 . 'for an example.)';
            throw new ApiException($msg);
        }
    }