a social event taking place in the grass.
A high school prom in a gym.
A gym is indoors.  Grass is found outdoors, so you cannot be in a gym and on grass at the same time.
exists e1 s1 x1 x2 x3 x4.(n_grass(x4) & a_topic(x1) & in(e1,x4) & Theme(e1,x3) & Actor(e1,x2) & v_take(e1) & n_place(x3) & (x1 = x2) & n_event(x1) & a_social(s1) & Theme(s1,x1))
exists s1 x1 x2 x3.(a_topic(x1) & in(x1,x3) & n_gym(x3) & n_prom(x1) & of(x1,x2) & n_school(x2) & a_high(s1) & Theme(s1,x1))