a skateboarder grinds a curb.
A skater falls down the stairs.
If a skater falls, he cant grind.
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_grind(e1) & n_curb(x2) & n_skateboarder(x1))
exists e1 x1 x2.(n_stair(x2) & down(e1,x2) & Actor(e1,x1) & v_fall(e1) & n_skater(x1))