Children on tricycles.
The children are riding bicycles.
Children riding bicycles can't be riding tricycles simultaneously.
exists x1 x2.(a_topic(x1) & on(x1,x2) & n_tricycle(x2) & n_child(x1))
exists e1 x1 x2.(n_child(x1) & Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_bicycle(x2))