Method mutable_key_comp

external/parallel_hashmap/btree.h:1960–1960

        const node_type *root() const { return std::get<2>(root_); }
        node_type *&mutable_root() noexcept { return std::get<2>(root_); }
        key_compare *mutable_key_comp() noexcept { return &std::get<0>(root_); }

        // The leftmost node is stored as the parent of the root node.
        node_type *leftmost() { return root()->parent(); }