A man in a helmet is riding a bike.
A bare-headed man is riding a bike.
A person can't simultaneously wear a helmet and not have one on.
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 s1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_bike(x2) & n_man(x1) & a_bare-headed(s1) & Theme(s1,x1))