code: @Override
  	public int indexOf(Pair<T, I> element) {
  		return matrix.indexOf(
  				transactionToIndex(element.transaction), 
  				itemToIndex(element.item));
  	}
docs: Provides position of element within the set.
  <p>
  It returns -1 if the element does not exist within the set.
  
  @param element
  element of the set
  @return the element position