People are at a party at night.
Someone is having a birthday party
Not all parties are birthday parties.
exists p1 x1 x2.(at(p1,x2) & n_night(x2) & exists x3.(at(x1,x3) & n_party(x3)) & n_people(x1))
exists e1 x1 x2 x3.(Theme(e1,x2) & Actor(e1,x1) & v_have(e1) & n_party(x2) & of(x2,x3) & n_birthday(x3) & n_person(x1))