- add progression
- add explanation/tutorial
- timed
- add column labels
- add spacing beyond X places
- add color and font
- optional super mario tyype sounds
- gamification via character upgrades
- processor graphic/animation
  - diff processors for different bits
- mobile
  - make autofocus work on quiz
  - remove "[enter]" from start button
  - make it all on one line 
    - make the font smaller or the div wider
  - put a 'submit' button in
  - add a range slider for easier input?
- make text inputs big enough for MAX_DIGITS_PROBLEMS and MAX_DIGITS_BITS
- show a message on submit
- show warning when specifying invalid number of bits or problems
- add contact info
- add 'about'
- add mailing list
  - prompt at the end of a quiz to sign up for it
- uncomment PDF part
- add PDF export in pure front end
- add animations
  - 'nuh-uh' shake for wrong answers
  - nice reward animations for right answers
  - make the placeholders become real text for a split second before starting
  - super mario land [pixellation animation](http://blog.swishscripts.com/2019/06/19/snes-mosaic-effect/) ("mosaic")
- add user accounts using Userbase
  - required for higher numbers
  - required for PDF output?
  - required after X uses?