Someone is swimming in a pool.
Someone skis down the mountain
The person is either swimming in a pool or skiing down the mountain, but not both.
exists e1 x1 x2.(in(e1,x2) & n_pool(x2) & Actor(e1,x1) & v_swim(e1) & n_person(x1))
exists x1 x2.(n_mountain(x2) & a_topic(x1) & down(x1,x2) & n_ski(x1))