A guard is on the look out while on duty.
Someone is keeping guard.
Someone is a guard. He is keeping guard so he is on duty.
exists p1 s1 x1 x2 x3.(n_look(x3) & on(p1,x2) & n_duty(x2) & r_while(p1) & a_out(s1) & Manner(p1,s1) & on(x1,x3) & n_guard(x1))
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_keep(e1) & n_guard(x2) & n_person(x1))