code: public IndexedSet<T> transactionsOf(I item) {
  		IndexedSet<T> res = allTransactions.empty();
  		res.indices().addAll(matrix.getCol(itemToIndex(item)));
  		return res;
  	}
docs: Lists all transactions involved with a specified item
  
  @param item
  the given item
  @return transactions involved with a specified item