constexpr iterator T_fill_insert(difference_type offset, size_type n, const T &x) {
            const auto old_size = size_;
            /* check if reallocation is needed */
            T_grow(size_ + n);
            size_ += n;
            /* get the iterator at offset */
            iterator pos = begin() + offset;
            /* we can move already initialized parts of memory
             * that we can move */
            std::move(pos,begin()+old_size-n,pos+n);
            std::uninitialized_move(pos+old_size-n, end(), end()-n);
            std::uninitialized_fill(pos, pos + n, x);
            return pos;
        }