Expression Tree diagrams form a Unital Magma - can we exploit that?

[igormoreno]

https://ncatlab.org/nlab/show/magma
https://en.wikipedia.org/wiki/Magma_(algebra)

A binary operation on a set S is a function (−)⋅(−):S×S→S from the Cartesian product S×S to S. A Magma (or binary algebraic structure, or, alternatively, a mono-binary algebra) (S,⋅) is a set equipped with a binary operation on it.
A Magma is called unital if it has a neutral element.

We can build trees by folding small trees together. The way to combine them is to join them by the roots. That binary operation is not associative but it has a neutral element: the empty tree. That works also for the graph-like representation of the expressions tree diagram, with the neutral element ExpTreeDiagram Set.empty Set.empty Nothing. The sum of diagrams is being implemented now in the pattern DiaBranch but maybe we can take advantage of this structural property somehow.