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3. Russell, B. The Principles of Mathematics. Cambridge, England: Allen & Unwin. 1967.
4. Church, A. A Formulation of the Simple Theory of Types. The Journal of Symbolic Logic, 1940, 5(2): 56–68.
5. Russell, B. “Appendix B: The Doctrine of Types,” in Bertrand Russell, The Principles of Mathematics, Cambridge: Cambridge University Press, 1903, 523–528.