A surfer is riding a wave.
The surfer can swim.
A surfer does not have to swim to be riding a wave.
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_wave(x2) & n_surfer(x1))
exists e1 p1 x1.(n_surfer(x1) & exists e2.(Actor(e2,x1) & v_swim(e2)) & Topic(e1,p1) & Actor(e1,x1) & v_can(e1))