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Michael Dummett, Elements of Intuitionism (1977), offers a clear presentation of the philosophic approach that demands constructibility in logical proofs. G.e. Hughes and M.J. Cresswell, An Introduction to Modal Logic (1968, reprinted 1989), treats operators acting on sentences in first-order logic (or predicate calculus) so that, instead of being interpreted as statements of fact, they become necessarily or possibly true or true at all or some times in the past, or they denote obligatory or permissible actions, and so on. Jon Barwise et al. (eds.), Handbook of Mathematical Logic (1977), provides a technical survey of work in the foundations of mathematics (set theory) and in proof theory (theories with infinitely long expressions). Elliott Mendelson, Introduction to Mathematical Logic, 3rd ed. (1987), is the standard text; and G. Kreisel and J.L. Krivine, Elements of Mathematical Logic: Model Theory (1967, reprinted 1971; originally published in French, 1967), covers all standard topics at an advanced level. A.S. Troelstra, Choice Sequences: A Chapter of Intuitionistic Mathematics (1977), offers an advanced analysis of the philosophical position regarding what are legitimate proofs and logical truths; and A.S. Troelstra and D. van Dalen, Constructivism in Mathematics, 2 vol. (1988), applies intuitionist strictures to the problem of the foundations of mathematics.