A person holding the leash of a dog as the dog sniffs the horse.
A person is riding a horse.
Just because the person is holding a leash of a dog sniffing at a horse does not mean the person is riding that horse.
exists e1 p1 x1 x2 x3 x4 x5.(n_horse(x5) & n_leash(x3) & a_topic(x1) & exists e2.(Theme(e2,x5) & Actor(e2,x4) & v_sniff(e2)) & n_dog(x4) & n_dog(x4) & as(e1,p1) & Theme(e1,x3) & Actor(e1,x2) & v_hold(e1) & of(x3,x4) & (x1 = x2) & n_person(x1))
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_horse(x2) & n_person(x1))