docs: function slowLoop(items, loopBody) {
  	return new Promise(f => {
  		let done = arguments[2] || f;
  		let idx = arguments[3] || 0;
  		let cb = items[idx + 1] ? () => slowLoop(items, loopBody, done, idx + 1) : done;
  		loopBody(items[idx], idx, cb);
  	});
  }