This example is shown in the design section. If there are questions I can flesh things out further here.
The trick is to define a data stream that invokes its actions in response to an action from anther source. Take the case of combining several min-bias events with a regular event. We will have two streams, the MC event stream which has a stop for every MC event. The second stream is one of min-bias events, and it has no stops per-see. The user first creates an action in the min-bias stream. The min bias action takes, as arguments, two edm events: the master MC event and a minbias event. The user's action does the work of folding the minbias into the MC event. The min-bias stream creates an event action in the event stream. When the event stream stops, the minbias action is called first. It then calls each of its actions several times (one for each event that is to be folded into the MC event). Once that is done, the rest of the user's event actions can execute with the modified MC event as data.
I'd draw some pictures, but I'm afraid I'm out of time.