Here are some pictures I made for myself trying to "feel" the logic of Tree Calculus's reduction rules: https://latypoff.com/tree-calculus-visualized/ — might be handy for other people who are visual thinkers.
That is very helpful. I particularly appreciate the carefully written text.
I think the second figure, captioned "Stem with a single leaf child" has a mistake, with the line down from the triangle descending to a square. But that square should be a circle.
