premise: A horse and rider are in midair as they leap over an obstacle in a wooded area with another photographer snapping pictures.
hypothesis: A person is riding a horse.
explanation_1: If a person is riding, he comes the rider.
explanation_2: Naturally, we assume that the rider who is performing such a stunt on a horse is a person.
explanation_3: The term rider indicates that someone is on something.  Since it says a horse and rider, it is inferred that the person is riding the horse.
premise_logic: exists p1 p2 x1 x2 x3.(exists e2 e3 e1 s1 x5 x6 x7 x8 x9.(in(e1,x6) & with(x6,x9) & n_picture(x9) & Actor(e3,x9) & v_snap(e3) & of(x9,x8) & n_photographer(x8) & -(x9 = x7) & n_picture(x7) & Actor(e2,x7) & v_snap(e2) & of(x7,x8) & n_area(x6) & a_wooded(s1) & Theme(s1,x6) & over(e1,x5) & n_obstacle(x5) & Actor(e1,x2) & v_leap(e1)) & n_thing(x2) & as(p1,p2) & exists x4.(in(x1,x4) & n_midair(x4)) & n_rider(x3) & n_horse(x2) & subset_of(x3,x1) & subset_of(x2,x1))
hypothesis_logic: exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_horse(x2) & n_person(x1))