struct dnode {
        constexpr dnode() = default;

        dnode(const dnode &) = delete;
        dnode &operator=(const dnode &) = delete;

        constexpr dnode(dnode &&a) noexcept
            : prev(a.prev)
            , next(a.next) {
                prev->next = this;
                next->prev = this;
        }
        constexpr dnode &operator=(dnode &&a) noexcept {
                unlink();

                next = a.next;
                prev = a.prev;

                prev->next = this;
                next->prev = this;

                return *this;
        }

        constexpr bool islinked() const noexcept { return next != this; }
        constexpr void unlink() noexcept {
                prev->next = next;
                next->prev = prev;
                next = prev = this;
        }

        ~dnode() noexcept { unlink(); }

private:
        friend struct dlist;
        friend struct backward_iterator;
        friend struct forward_iterator;

        dnode *prev = this, *next = this;
};