// ..., dn-1),
  //   - if not block sparse, the traversal_order is just a permutation of (d0,
  //     ..., dn-1). For example, a 2-D matrix stored in row-major order would
  //     have traversal_order = (d0, d1).
  //   - if block sparse with a k-dimensional block (0 <= k <= n), the
  //     traversal_order has n + k elements. The first n elements are still a