people are at a costume party.
There are people in costume at a party.
There are people at a party refers to people are at a costume
exists p1 x1.(exists s1 x2.(at(x1,x2) & n_party(x2) & a_costume(s1) & Theme(s1,x2)) & n_people(x1))
exists p1 x1 x2 x3.(in(p1,x2) & at(x2,x3) & n_party(x3) & n_costume(x2) & exists x4.((x1 = x4) & n_people(x4)))