Other resources folks might want to check out are:
+ David Harel's original paper on the statecharts formalism (which Sketch.systems implements): http://www.inf.ed.ac.uk/teaching/courses/seoc/2005_2006/reso...
+ Ian Horrock's "Constructing the User Interface with Statecharts" book: https://amzn.to/2sT1e4x
It's also worth noting that Sketch.systems doesn't do any kind of formal analysis or checking of invariants.
If you're interested in that sort of thing, look into TLA+ (video intro: https://lamport.azurewebsites.net/video/videos.html) or Alloy (http://alloytools.org/).
!? Do you know the phrase "bury the lede" (sic)!? ;-)
This is cool, this is something.