code: public int fastUnorderedRemove(
          @DoNotSub final int index)
      {
          checkIndex(index);
  
          final int value = elements[index];
          elements[index] = elements[--size];
  
          return value;
      }
docs: Removes element at index, but instead of copying all elements to the left,
  it replaces the item in the slot with the last item in the list. This avoids the copy
  costs at the expense of preserving list order. If index is the last element it is just removed.
  
  @param index of the element to be removed.
  @return the existing value at this index.
  @throws IndexOutOfBoundsException if index is out of bounds.