A skier on a slope.
A person is skiing.
When a skier goes on a slope, they are there to go skiing.
exists x1 x2.(a_topic(x1) & on(x1,x2) & n_slope(x2) & n_skier(x1))
exists p1 x1.(exists x2.((x1 = x2) & n_skiing(x2)) & n_person(x1))