[Twey]

When you can build down as well as up (https://ngnghm.github.io/blog/2015/08/24/chapter-4-turtling-...) — or, equivalently, do away with the up/down orientation entirely, you can have your cake and eat it too.

And as a bonus if you control both slices of bread it's much easier to change the sandwich filling as well! (Though if the original sandwich-builder wasn't careful you might find some sticky residue left over on your bread… maybe someone should take this metaphor away before I do more damage.)