A man in a helmet is riding a bike.
A man in a helmet riding his mountain bike.
A man doesn't have to ride a mountain bike, it could be any kind of bike.
exists e1 x1 x2 x3.(Theme(e1,x3) & Actor(e1,x1) & v_ride(e1) & n_bike(x3) & in(x1,x2) & n_helmet(x2) & n_man(x1))
exists e1 x1 x2 x3 x4 x5.(a_topic(x1) & in(x1,x2) & Theme(e1,x4) & Actor(e1,x3) & v_ride(e1) & n_bike(x4) & of(x4,x5) & n_mountain(x5) & of(x4,x1) & n_male(x1) & (x2 = x3) & n_helmet(x2) & n_man(x1))