[sestep]

Yeah, exactly! Also, possibly a third idea of Penrose is the way the selector matching blocks in the Style language can be used to construct the constrained optimization problem as you mention. This is inspired by CSS, but there are a lot of differences too.

With regard to the separation of substance and style, I think that idea can be profitably applied in other settings as well. For instance, Graphviz, tikz-cd, and Mermaid are all fairly declarative. But also, I feel like this idea could be even more profitably applied by building a diagramming library inside of a general-purpose programming language like Python.

For instance, take a look at the Substance code for our quaternion multiplication table example: https://penrose.cs.cmu.edu/try/?examples=group-theory/quater... It's about 83 lines of code. In contrast, by really taking the separation idea seriously, one can write a generic Python function for creating Cayley table diagrams, after which that particular example only takes about 5 lines of code: https://github.com/samestep/diagrams/blob/750f7a544635a6fd9f...