A cowboy is holding the reins of his horse as he poses for a picture.
A cowboy sits on his horse
A cowboy does not have to sit on the horse to hold the reins. A cowboy who sits on his  horse may not be holding the reins.
exists e1 p1 x1 x2 x3.(n_male(x2) & n_male(x2) & n_horse(x3) & of(x3,x2) & n_rein(x2) & exists e2 x4.(for(e2,x4) & n_picture(x4) & Actor(e2,x2) & v_pose(e2)) & as(e1,p1) & Theme(e1,x2) & Actor(e1,x1) & v_hold(e1) & of(x2,x3) & n_cowboy(x1))
exists e1 x1 x2.(on(e1,x2) & n_horse(x2) & of(x2,x1) & n_male(x1) & Actor(e1,x1) & v_sit(e1) & n_cowboy(x1))