A boy carries a box across a bridge.
Someone is carrying something.
A boy is someone.  A box could be something.
exists e1 x1 x2 x3.(Theme(e1,x2) & Actor(e1,x1) & v_carry(e1) & across(x2,x3) & n_bridge(x3) & n_box(x2) & n_boy(x1))
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_carry(e1) & n_thing(x2) & n_person(x1))