 The class of terms is the smallest set containing the numerals, variables \(x_0,x_1, \ldots\) and closed under the operations \(t \mapsto s(t)\) and \(t_1,\ldots,t_n \mapsto \psi^n_i(t_1,\ldots,t_n)\) where \(t,t_1,\ldots,t_n\) are terms and \(\psi^n_i\) is a primitive \(n\)-ary functional symbol.