docs: public static @Nonnull <U> U lookupSingleton(Class<U> type) {
          ExtensionList<U> all = lookup(type);
          if (all.size() != 1) {
              throw new IllegalStateException("Expected 1 instance of " + type.getName() + " but got " + all.size());
          }
          return all.get(0);
      }