Question

David Turner introduced three new examples of these objects to create a bracket abstraction algorithm that has n squared output size instead of n cubed. By the Curry-Howard correspondence, these objects are linked with axiom schemes in a Hilbert-style (-5[1])deductive system. These objects are represented by kestrels, starlings, warblers, and other birds in the Raymond Smullyan puzzle book To Mock (-5[1])a Mockingbird. A basis of four of these objects representing composing, permuting, discarding, and duplicating is used by the BCKW system, which is equivalent in power to the (*) SKI (“S-K-I”) calculus that only uses three of them. (-5[1])These objects are equivalent to lambda expressions with no free variables. In lambda calculus, “fixed-point” kinds of these higher-order functions (10[1])are used to express recursion. For 10 points, the startup accelerator that hosts Hacker News (10[1])is named for the “Y” type of what object? ■END■ (10[2])

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