A man riding a dirt bike
A biker is racing his dirt bike on a course.
Riding doesn't imply racing. The fact that man riding a dirt bike doesn't imply that he is racing on a course.
exists e1 x1 x2 x3 x4.(a_topic(x1) & Theme(e1,x3) & Actor(e1,x2) & v_ride(e1) & n_bike(x3) & of(x3,x4) & n_dirt(x4) & (x1 = x2) & n_man(x1))
exists e1 x1 x2 x3 x4.(on(e1,x4) & n_course(x4) & Theme(e1,x2) & Actor(e1,x1) & v_race(e1) & n_bike(x2) & of(x2,x3) & n_dirt(x3) & of(x2,x1) & n_male(x1) & n_biker(x1))