Function join

fem/tensorcoefficient.cpp:190–196

        }

        size_t join(const FlatArray<size_t> full_indices, const MultiIndex &multi_idx) {
            const auto &strides = multi_idx.Strides();
            size_t res = 0;
            for (size_t i: Range(multi_idx.Size()))
                res += full_indices[multi_idx[i].pos] * strides[i];
            return res;
        }

        Array<size_t> split(size_t full_index, const MultiIndex &multi_idx) {
            const auto &strides = multi_idx.Strides();