 Once again, \(A(x/t)\) is the result of replacing all occurrences of individual constant \(t\) in \(A\) by the variable \(x\). If there are no such occurrences, then \(A(x/t)\) is just \(A\). In (A1) the variable \(x\) is not free in \(A\) (since otherwise \(A\) would be an open formula and formulas of \(\bL\) are closed). However, \(x\) may be free in \(A\) or \(B\) in (A2) and in \(A\) in (A3) and (A4).