The boy is skateboarding on the bench by the fence.
A boy is skateboarding in a competition.
A boy can be skateboarding on a bench and yet not be part of any competition.
exists e1 x1 x2 x3.(n_fence(x3) & n_bench(x2) & n_boy(x1) & by(e1,x3) & on(e1,x2) & Actor(e1,x1) & v_skateboard(e1))
exists e1 x1 x2.(in(e1,x2) & n_competition(x2) & Actor(e1,x1) & v_skateboard(e1) & n_boy(x1))