Harel's StateCharts offer wonderful insights into 

• diagrammatic programming
• isolation (encapsulation) of state
• isolation of control flow.

My slide deck and voice-over from a presentation^[1] is here:

https://computingsimplicity.neocities.org/Statecharts%20presentation%202020.pdf

https://soundcloud.com/user-408358553/statechartsaudio

and, my annotations for every diagram in the paper are here:

https://computingsimplicity.neocities.org/Statecharts%20annotations%202020.pdf

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[1] Papers We Love, Toronto, August 2019