A boy on a BMX bike
The rollerblader is a boy.
A boy can not be rollerblading and riding a bike at the same time.
exists x1 x2 x3.(a_topic(x1) & on(x1,x2) & n_bike(x2) & (x2 = x3) & ne_org_bmx(x3) & n_boy(x1))
exists p1 x1.(n_rollerblader(x1) & exists x2.((x1 = x2) & n_boy(x2)))