A person stunt riding on a BMX type of bicycle.
Nobody is riding
If a person is stunt riding, then it cannot be claimed that nobody is riding.
exists x1 x2 x3 x4 x5.(a_topic(x1) & on(x1,x3) & of(x3,x5) & n_bicycle(x5) & n_type(x3) & (x3 = x4) & ne_org_bmx(x4) & n_stunt(x1) & Time(x1,x2) & n_riding(x2) & n_person(x1))
-exists e1 x1.(Actor(e1,x1) & v_ride(e1) & n_person(x1))