person skiing off a mountain
There is no snow on the mountain the person is on
One cannot be skiing off a mountain with no snow on it.
exists x1 x2 x3 x4.(a_topic(x3) & Time(x3,x4) & n_skiing(x4) & n_person(x3) & off(x2,x1) & n_person(x2) & n_mountain(x1))
exists p1 x1 x3 x4.(n_person(x4) & n_mountain(x3) & -exists p2 x2.((x1 = x2) & on(x2,x3) & exists x5.(on(x4,x5) & (x3 = x5)) & n_snow(x2)))