Total sum of a participant in every event

Rather than just have the totals of a participant in one event, you could sum up all the debts that each participant owe in every event he participates.

For example: If John owes me 10€ during the event "Holidays" and another 20€ during the event "Party", it should be possible to see the total debt (30€) of both events for this participant. However I think one can only see the debt of a participant in each event, but not the total of every events in which he participates.