code: public IndexedSet<T> involvedTransactions() {
  		IndexedSet<T> res = allTransactions.empty();
  		res.indices().addAll(matrix.involvedRows());
  		return res;
  	}
docs: Gets the set of transactions in {@link #allTransactions()} that contains
  at least one item
  
  @return the set of transactions in {@link #allTransactions()} that
  contains at least one item