docs: function calcToRemove(a, b) {
  	var subLengths = 0;
  	subLengths += a.height === 0 ? 0 : sumOfLengths(a);
  	subLengths += b.height === 0 ? 0 : sumOfLengths(b);
  
  	return (a.length + b.length) - (Math.floor((subLengths - 1) / M) + 1);
  }