A young woman is sitting on the sidewalk sketching a building
Nobody is sitting.
Nobody sitting totally contradicts the woman mentioned being siting in the first sentence.
exists e2 e1 s1 x1 x2 x3.(n_sidewalk(x2) & for(e1,e2) & Theme(e2,x3) & Actor(e2,x1) & v_sketch(e2) & n_building(x3) & on(e1,x2) & Actor(e1,x1) & v_sit(e1) & n_woman(x1) & a_young(s1) & Theme(s1,x1))
-exists e1 x1.(Actor(e1,x1) & v_sit(e1) & n_person(x1))