docs: function saveSlot(aList, bList, index, slot) {
  	setEither(aList, bList, index, slot);
  
  	var isInFirst = (index === 0 || index === aList.sizes.length);
  	var len = isInFirst ? 0 : getEither(aList.sizes, bList.sizes, index - 1);
  
  	setEither(aList.sizes, bList.sizes, index, len + length(slot));
  }