I've been watching my wife teach the kids second-grade math for a few weeks now with the glimmer of an idea in my head that just came out.
The tree notation for addition and subtraction shows the symmetry between them, and also the symmetry between operands.
I know it's valuable, because it got instantly adopted in lessons, where most of my ideas get gently humored and booted out of the room.
I'd never have the sense something like this was valuable before hanging out here ❤️