A man is operating a piece of machinery.
Some men are working on a machine
A man operating a piece of machinery does not imply that some men are working on a machine.
exists e1 x1 x2 x3.(Theme(e1,x2) & Actor(e1,x1) & v_operate(e1) & of(x2,x3) & n_machinery(x3) & n_piece(x2) & n_man(x1))
exists e1 x1 x2.(on(e1,x2) & n_machine(x2) & Actor(e1,x1) & v_work(e1) & n_man(x1))