A man wakeboarding
someone is wakeboarding
A man is a person, and "someone" means "some person".
exists e1 x1 x2.(a_topic(x1) & Actor(e1,x2) & v_wakeboard(e1) & (x1 = x2) & n_man(x1))
exists e1 x1.(Actor(e1,x1) & v_wakeboard(e1) & n_person(x1))