code: @Override
      public T set(int index, T elem) {
          /* Find out where to look. */
          final int offset = computeOffset(index);
          final int arrIndex = computeIndex(index);
  
          /* Cache the value there and write the new one. */
          T result = mArrays[offset][arrIndex];
          mArrays[offset][arrIndex] = elem;
  
          /* Hand back the old value. */
          return result;
      }
docs: Sets the element at the specified position to the indicated value.
  If the index is out of bounds, throws an IndexOutOfBounds exception.
  
  @param index The index at which to set the value.
  @param elem  The element to store at that position.
  @return The value initially at that location.
  @throws IndexOutOfBoundsException If index is invalid.