party

part of partition that contains specified element

letting P be partition({1,2},{3},{4,5,6})
find c : set of int(1..6) such that c = party(4,P) ${4,5,6}

This syntax is finding the part of the partition that contains 4. So the result is {4,5,6} because 4 is in the {4,5,6}.

See it demonstrated here.