A girl on a bike stops to look back.
A boy looks ahead on his skateboard.
The original subject was a girl on a bike looking back, while the subject changed to a male, looking ahead, on a skateboard.
exists e1 p1 x1 x2.(Topic(e1,p1) & Actor(e1,x1) & v_stop(e1) & exists e2 p2.(exists s1.(Theme(s1,x1) & a_back(s1)) & Topic(e2,p2) & Actor(e2,x1) & v_look(e2)) & on(x1,x2) & n_bike(x2) & n_girl(x1))
exists e1 s1 x1 x2.(a_ahead(s1) & Manner(e1,s1) & on(e1,x2) & n_skateboard(x2) & of(x2,x1) & n_male(x1) & Actor(e1,x1) & v_look(e1) & n_boy(x1))