A man walking by a stop sign.
A woman waiting at a stop sign
The two subjects are different. The subject of the first sentence is a man while the subject of the second sentence is a woman.
exists x1 x2 x3 x4.(a_topic(x1) & by(x1,x3) & n_sign(x3) & of(x3,x4) & n_stop(x4) & n_walking(x1) & of(x1,x2) & n_man(x2))
exists e1 x1 x2 x3 x4.(a_topic(x1) & at(e1,x3) & n_sign(x3) & of(x3,x4) & n_stop(x4) & Actor(e1,x2) & v_wait(e1) & (x1 = x2) & n_woman(x1))