code: public function addLoader($extensions, $loaderClass)
  	{
  		foreach ((array) $extensions as $extension) {
  			$this->extensions[] = $extension;
  			if (is_string($loaderClass)) {
  				$this->loaderClasses[$extension] = $loaderClass;
  			} elseif ($loaderClass instanceof LoaderInterface) {
  				$this->loaders[$extension] = $loaderClass;
  			}
  		}
  	}
docs: Add a loader.
  
  @param  string $extensions
  @param  string|LoaderInterface $loaderClass
  
  @return void