Man on bike in mountains
Nobody is on a bike
A man is on the bike, so it is wrong to say nobody is on the bike.
exists x1 x2 x3.(a_topic(x1) & on(x1,x2) & in(x2,x3) & n_mountain(x3) & n_bike(x2) & n_man(x1))
-exists p1 x1.(exists x2.(on(x1,x2) & n_bike(x2)) & n_person(x1))