A boy mountain bikes down a trail.
A girl motor bikes down a country lane.
There is either a boy doing something or a girl doing something.
exists x1 x2 x3 x4.(a_topic(x1) & down(x1,x4) & n_trail(x4) & n_bike(x1) & of(x1,x3) & n_mountain(x3) & of(x1,x2) & n_boy(x2))
exists x1 x2 x3 x4 x5.(a_topic(x1) & down(x1,x4) & n_lane(x4) & of(x4,x5) & n_country(x5) & n_bike(x1) & of(x1,x3) & n_motor(x3) & of(x1,x2) & n_girl(x2))