1. Beth, E.W., Formal Methods, An Introduction to Symbolic Logic and to the Study of Effective Operations in Arithmetic and Logic, Dordrecht 1962
2. For more details, cf. Beth, E.W., Formal Methods
3. Beth, E.W., The Foundations of Mathematics (Studies in Logic), Amsterdam 1959
4. On these matters, cf. Braffort, P., D. Hirschberg and J. Mommens, La programmation des problèmes non numériques (mimeographed)
5. Couturat, L., (ed.), Opuscules et fragments inédits de Leibniz, Paris 1903, Index nominum et rerum s.v. “Machina”, p. 650