A person on a tricycle.
A person riding a tricyle.
if there is a person on a tricycle then the person is riding a tricycle.
exists x1 x2.(a_topic(x1) & on(x1,x2) & n_tricycle(x2) & n_person(x1))
exists e1 x1 x2 x3.(a_topic(x1) & Theme(e1,x3) & Actor(e1,x2) & v_ride(e1) & n_tricyle(x3) & (x1 = x2) & n_person(x1))