code: @Override
  	public Pair<T, I> get(int index) {
  		return indexToPair(matrix.get(index));
  	}
docs: Gets the <code>i</code><sup>th</sup> element of the set
  
  @param index
  position of the element in the sorted set
  @return the <code>i</code><sup>th</sup> element of the set
  @throws IndexOutOfBoundsException
  if <code>i</code> is less than zero, or greater or equal to
  {@link #size()}