A person riding a rodeo bull.
A cowboy is riding a bull.
Just because a person riding a bull doesn't mean he's a cowboy.
exists e1 x1 x2 x3 x4.(a_topic(x1) & Theme(e1,x3) & Actor(e1,x2) & v_ride(e1) & n_bull(x3) & of(x3,x4) & n_rodeo(x4) & (x1 = x2) & n_person(x1))
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_bull(x2) & n_cowboy(x1))