A boy in a gray shirt is skateboarding.
A boy is outside riding his bike.
One can not be skateboarding and riding a bike simultaneously.
exists e1 s1 x1 x2.(Actor(e1,x1) & v_skateboard(e1) & in(x1,x2) & n_shirt(x2) & a_gray(s1) & Theme(s1,x2) & n_boy(x1))
exists e1 s1 x1 x2.(a_outside(s1) & Manner(e1,s1) & Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_bike(x2) & of(x2,x1) & n_male(x1) & n_boy(x1))