A group of people gathered for an event
Group at an concert
The event is not necessarily a concert.
exists e1 x1 x2 x3.(for(e1,x3) & n_event(x3) & Actor(e1,x1) & v_gather(e1) & of(x1,x2) & n_people(x2) & n_group(x1))
exists s1 x1.(ne_org_group(s1) & a_topic(s1) & at(s1,x1) & n_concert(x1))