protected function _checkManifestIsValid( $location, array $manifest )
	{
		if ( isset( $this->_extensions[$manifest['name']] ) ) {
			$location2 = $this->_extensions[$manifest['name']]['location'];
			$msg = 'Extension "%1$s" exists twice in "%2$s" and in "%3$s"';
			throw new Exception( sprintf( $msg, $manifest['name'], $location, $location2 ) );
		}

		if ( !isset( $manifest['depends'] ) || !is_array( $manifest['depends'] ) ) {
			throw new Exception( sprintf( 'Incorrect dependency configuration in manifest "%1$s"', $location ) );
		}
	}