Method seek

common/unordered_inline_set.h:116–118

        SparseArray* _a = nullptr;
        size_t _i = 0;
        void seek(size_t d) {
            _i = _a->seek_existance(_i + d, d);
        }
    public:
        iterator() = default;
        iterator(SparseArray* a, size_t i = 0) : _a(a), _i(i) { }