- explicit maximal_subsets_iterator(const EventSet& events, std::optional<node_filter_function> filter = std::nullopt,
- std::optional<size_t> maximum_subset_size = std::nullopt);
+ explicit maximal_subsets_iterator(const EventSet& events,
+ const std::optional<node_filter_function>& filter = std::nullopt,
+ std::optional<size_t> maximum_subset_size = std::nullopt);