A street musician playing his guitar on the street.
A musician is not very good at playing his instrument.
A musician playing guitar  does not imply that he is not very good at playing or good at playing.
exists e1 x1 x2 x3 x4.(a_topic(x1) & Theme(e1,x4) & Actor(e1,x3) & v_play(e1) & on(x4,x2) & n_street(x2) & n_street(x2) & n_guitar(x4) & of(x4,x1) & n_male(x1) & (x1 = x3) & n_musician(x1) & of(x1,x2))
exists x1 x3.(-exists e1 s1 x2.(r_very(s1) & at(s1,e1) & Theme(e1,x3) & Actor(e1,x2) & v_play(e1) & Theme(s1,x1) & a_good(s1)) & n_instrument(x3) & of(x3,x1) & n_male(x1) & n_musician(x1))