A boy on a BMX bike
The biker is a male youngster.
Since the person on the bike is a boy, he has has to be a male youngster.
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_biker(x1) & exists s1 x2.((x1 = x2) & n_youngster(x2) & a_male(s1) & Theme(s1,x2)))