Two doctors perform an operation.
The doctors are operating.
doctors perform an operation. is same as the doctors are operating.
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_perform(e1) & n_operation(x2) & n_doctor(x1) & card_eq_2(x1))
exists e1 x1.(n_doctor(x1) & Actor(e1,x1) & v_operate(e1))