 If \(\phi_{\tau}^{n}\) is an n-place propositional function variable of r-type \((\tau_1 \ldots, \tau_n) /k\) and \(x_{i} \) for \( ( 0 \leq i \leq n ) \) are terms of of r-types \(\tau_1 \ldots, \tau_n\), respectively, then \(\phi (x_1, \ldots x_n)\) is a wff.