These approaches may be nice to demonstrate the concept in brief but I'm a bit sad the article didn't take the opportunity to go into a design that only stores

These approaches may be nice to demonstrate the concept in brief but I'm a bit sad the article didn't take the opportunity to go into a design that only stores the triangular data since it's pretty trivial to overload operators in C++. If this is meant to be a demonstration of the performance advantage of mdspan over nested vector creation (which certainly is the case for large multidimensional arrays) it'd be good to dial that up.