public function registerBackend(AbstractBackend $backend)
	{
		if (in_array($backend, $this->backends, TRUE)) {
			throw new InvalidArgumentException('Duplicate backend: '.get_class($backend));
		} else {
			$this->backends[] = $backend;

			if ($this->debug_logger) {
				$backend->setDebugLogger($this->debug_logger);
			}
		}
		return $this;
	}