docs: function unfilled(cube) {
  	const tuples = Cube.cartesian(cube);
  	const unfilled = [];
  	tuples.forEach(tuple => {
  		const members = this.dice(tuple).getFacts(tuple);
  		if (members.length === 0) {
  			unfilled.push(tuple)
  		}
  	});
  	return unfilled;
  }