A boy does a cartwheel on a rock cliff.
A boy is doing a cartwheel in front of a crowd of people.
Cliffs can be empty.  There is no mention of a crowd of people.
exists e1 x1 x2 x3 x4.(on(e1,x3) & n_cliff(x3) & of(x3,x4) & n_rock(x4) & Theme(e1,x2) & Actor(e1,x1) & v_do(e1) & n_cartwheel(x2) & n_boy(x1))
exists e1 x1 x2 x3 x4 x5.(Theme(e1,x2) & Actor(e1,x1) & v_do(e1) & in(x2,x3) & of(x3,x4) & of(x4,x5) & n_people(x5) & n_crowd(x4) & n_front(x3) & n_cartwheel(x2) & n_boy(x1))