docs: function interval_diff(progression1, progression2, interval) {
  	var i = numeral_intervals[numerals.indexOf(progression1)],
  		j = numeral_intervals[numerals.indexOf(progression2)];
  		acc = 0;
  	if (j < i) {
  		j += 12;
  	}
  	while(j - i > interval) {
  		acc -= 1;
  		j -= 1;
  	}
  	while(j - i < interval) {
  		acc += 1
  		j += 1
  	}
  	return acc;
  }