 is another example of many-sorted structure. This structure includes the domain of individuals \(\mathbf{A}\) as well as a family of second order domains of n-ary relations on individuals, \(\wp (\mathbf{A}^{n})\), for each natural number \(n\). The relations and functions in \(\langle f^{\mathcal{A}}\rangle _{f\in \OperSym}\) are first-order relations and functions defined over the universe of individuals (for more details about second-order logic, see the entry on  second-order and higher-order Logic).