A dog chases after a yellow ball.
the dog is chasing a red ball
if a dog chases after a yellow ball then it can't be chasing a red ball.
exists e1 s1 x1 x2.(after(e1,x2) & n_ball(x2) & a_yellow(s1) & Theme(s1,x2) & Actor(e1,x1) & v_chase(e1) & n_dog(x1))
exists e1 s1 x1 x2.(n_dog(x1) & Theme(e1,x2) & Actor(e1,x1) & v_chase(e1) & n_ball(x2) & a_red(s1) & Theme(s1,x2))