  The language to be used is a straightforward first order extension of the propositional modal language. There is an infinite list of object variables, \(x, y, x_{1}, x_{2},\ldots\), and a list of relation symbols, \(R, P, P_{1}, P_{2},\ldots\), of all arities. Among these is the one-place symbol \(E\) and the two-place symbol =. Constant and function symbols could be added, but let’s keep things relatively simple, along with simply relative. If \(x_{1}, \ldots ,x_{n}\) are object variables and \(P\) is an \(n\)-place relation symbol, \(P(x_{1}, \ldots ,x_{n})\) is an atomic formula. We’ll write \(x = y\) in place of = (x,y). More complex formulas are built up using propositional connectives, modal operators, and quantifiers, \(\forall\) and \(\exists\), in the usual way. Free and bound occurrences of variables have the standard characterization.