The boy is skateboarding on the stairs.
The boy is on the stairs.
Boy is skateboarding on the stairs implies boy is on stairs
exists e1 x1 x2.(n_stair(x2) & n_boy(x1) & on(e1,x2) & Actor(e1,x1) & v_skateboard(e1))
exists p1 x1 x2.(n_stair(x2) & n_boy(x1) & on(x1,x2))