Four people are on a ski slope.
The ski slope is empty.
four people present at the ski slope, means it is impossible for the ski slope to be empty.
exists p1 x1.(exists x2 x3.(on(x1,x2) & n_slope(x2) & of(x2,x3) & n_ski(x3)) & n_people(x1) & card_eq_4(x1))
exists s1 x1 x2.(n_slope(x1) & of(x1,x2) & n_ski(x2) & Theme(s1,x1) & a_empty(s1))