People riding horses on the beach.
A girl drives on the highway.
People means more than one person and a girl is one person, so it's either one or the other. People cannot be riding horses on the beach and driving on the highway at the same time.
exists e1 x1 x2 x3 x4.(n_beach(x4) & a_topic(x1) & on(e1,x4) & Theme(e1,x3) & Actor(e1,x2) & v_ride(e1) & n_horse(x3) & (x1 = x2) & n_people(x1))
exists x1 x2 x3.(n_highway(x3) & a_topic(x1) & on(x1,x3) & n_drive(x1) & of(x1,x2) & n_girl(x2))