A person on a snowmobile.
A person is skiing.
A person cannot be on a snowmobile and skiing at the same time.
exists x1 x2.(a_topic(x1) & on(x1,x2) & n_snowmobile(x2) & n_person(x1))
exists p1 x1.(exists x2.((x1 = x2) & n_skiing(x2)) & n_person(x1))