People are dancing at a party.
The people are celebrating at a birthday party.
People can be dancing without celebrating something.
exists e1 x1 x2.(at(e1,x2) & n_party(x2) & Actor(e1,x1) & v_dance(e1) & n_people(x1))
exists e1 x1 x2 x3.(n_people(x1) & at(e1,x2) & n_party(x2) & of(x2,x3) & n_birthday(x3) & Actor(e1,x1) & v_celebrate(e1))