match_key_events.js

  function getCharInRange(first, last) {
    const charactersInRange = R.compose(R.map(keycode), R.range)
    return charactersInRange(first, last)
  }
  
  const keystroke$ = getCharInRange(65, 91).reduce((acc$, char) => 
    acc$.merge(Observable.zip(
      DOM
        .select(':root')
        .events('keydown')
        .map(eventToObject)
        .filter(obj => obj.key === char),
      DOM
        .select(':root')
        .events('keyup')
        .map(eventToObject)
        .filter(obj => obj.key === char)
  )), Observable.empty())