Girls are playing volleyball.
Boys are playing volleyball.
Only the boys or the girls can play volleyball, unless the two groups are either playing independently of one another or are playing together without separating by gender.
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_play(e1) & n_volleyball(x2) & n_girl(x1))
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_play(e1) & n_volleyball(x2) & n_boy(x1))