1. See alsoD. Kalish andR. Montague,Logic, Techniques of Formal Reasoning, New York 1964.
2. P. Bernays andA. A. Fraenkel,Axiomatic Set Theory, Amsterdam 1958.
3. We assume that no bound variable occuring in e1 or ... or in e k or in ek+1 occurs in e.
4. This does not hold in many theories of descriptions. F.i. the models for the language with iotaoperator contain, according to the theory of Montague-Kalish, a distinguished element denoted by all so called improper descriptions.
5. SeeK. Ajdukiewicz,Die Definition, Actes du Congrès International de Philosophie Scientifique (Sorbonne 1935), vol. III, 1–7, Paris 1936.