FWIW, Brown University was doing research in this *Pad field (stylus-based input apps for domain specific notations) in the late 90s/2000s. I was a member of this group for a brief period of time. It's really exciting to see a resurgence in high quality pen computing with affordable, entry-level consumer hardware like the Surface 3.

Music Notepad (original): http://graphics.cs.brown.edu/research/music/home.html

Music Notepad (tabletpc): http://graphics.cs.brown.edu/research/music/tpc.html

Mathpad: (Commercialized) http://www.fluiditysoftware.com/ https://www.youtube.com/watch?v=BAFGONn4KoQ

Chempad: http://pen.cs.brown.edu/research.html#chempad