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Probably means the 3 irreducable primitives in LC: applications, abstractions, and "variables" (ie. attribute identifiers)

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In this context, it’s more likely the 3 basic primitive recursive functions: constant, successor, projection. https://en.wikipedia.org/wiki/Primitive_recursive_function#D...

Those functions compose into programs that halt (if I read the linked Wiki right). The LC encompasses those programs plus more programs, which cannot be shown to halt.

These three? https://en.wikipedia.org/wiki/SKI_combinator_calculus

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