The boy is skateboarding on the stairs.
The boy is skateboarding down the rail of the stairs.
There is no evidence that the stairs has rails or that the boy was skateboarding down them.
exists e1 x1 x2.(n_stair(x2) & n_boy(x1) & on(e1,x2) & Actor(e1,x1) & v_skateboard(e1))
exists e1 s1 x1 x2 x3.(n_stair(x3) & n_rail(x2) & n_boy(x1) & a_down(s1) & Manner(e1,s1) & Theme(e1,x2) & Actor(e1,x1) & v_skateboard(e1) & of(x2,x3))