for example). In some sources this is a set of sets, while other sources allow it to be a multiset of sets, with some sets repeated.An [[indexed family]] of sets is by definition a set-valued function (that is, it is a function that assigns a set to every element in its domain) whose domain is called its [[index set]] (and elements of its domain are called indices).

There are two subtly different definitions for when a family of sets is called pairwise