fixes #63

lib/CombinedContainer.php

@@ -26,7 +26,7 @@ final class CombinedContainer implements ContainerInterface
         throw new NotFoundException();
     }
 
-    public function has(string $id)
+    public function has(string $id): bool
     {
         foreach ($this->containers as $container) {
             if (!$container->has($id)) {