premise: Three boys in swimming trunks walk upstream.
hypothesis: Some boys are in swimwear are walking in the stream.
explanation_1: The boys heading upstream could be walking next to the water, not necessarily in the stream.  There is no way to be sure the body of water is a stream since the term upstream can describe moving opposite to the flow of water in any body of water with a current such as a river, brook, stream, tributary, etc.
premise_logic: exists e1 x1 x2 x3 x4.(Theme(e1,x4) & Actor(e1,x1) & v_walk(e1) & n_upstream(x4) & in(x1,x2) & n_trunk(x2) & of(x2,x3) & n_swimming(x3) & n_boy(x1) & card_eq_3(x1))
hypothesis_logic: exists e1 p1 x1 x2 x3.(n_stream(x3) & in(e1,x3) & Actor(e1,x1) & v_walk(e1) & exists x4.(in(x2,x4) & n_swimwear(x4)) & (x1 = x2) & n_boy(x1))
premise_sim: exists e1 x1 x2 x3 x4.(v_walk(e1) & of(x2,x3) & n_upstream(x4) & n_trunk(x2) & n_swimming(x3) & n_boy(x1) & in(x1,x2) & card_eq_3(x1) & Theme(e1,x4) & Actor(e1,x1))
hypothesis_sim: exists e1 x1 x2 x3.(v_walk(e1) & n_stream(x3) & n_boy(x1) & in(e1,x3) & exists x4.(n_swimwear(x4) & in(x2,x4)) & Actor(e1,x1) & (x1 = x2))