A man on a motorcycle with no shirt on at a bike event.
A man riding a motorcycle.
The fact that a man is on a motorcycle doesn't imply he is riding a motorcycle.
exists x1 x2 x3.(-exists e1 x4 x5 x6.(r_.(e1) & r_on(e1) & at(e1,x5) & n_event(x5) & of(x5,x6) & n_bike(x6) & Theme(e1,x2) & Actor(e1,x1) & v_event(e1) & on(x2,x3) & with(x3,x4) & n_shirt(x4)) & n_motorcycle(x3) & n_man(x2) & n_thing(x1))
exists e1 x1 x2 x3.(a_topic(x1) & Theme(e1,x3) & Actor(e1,x2) & v_ride(e1) & n_motorcycle(x3) & (x1 = x2) & n_man(x1))