A surfer riding the wave.
A person surfing.
Riding the wave is called surfing.
exists e1 x1 x2 x3.(n_wave(x3) & a_topic(x1) & Theme(e1,x3) & Actor(e1,x2) & v_ride(e1) & (x1 = x2) & n_surfer(x1))
exists x1 x2.(a_topic(x1) & Time(x1,x2) & n_surfing(x2) & n_person(x1))