A man is operating a piece of machinery.
At his job the man operates a piece of machinery.
A man operating a piece of machinery does not necessarily imply being at his job.
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 x3 x4.(n_man(x1) & at(e1,x4) & n_job(x4) & of(x4,x2) & n_male(x2) & Theme(e1,x2) & Actor(e1,x1) & v_operate(e1) & of(x2,x3) & n_machinery(x3) & n_piece(x2))