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Section1 adapts Tom Leinster, Basic Category Theory, arXiv:1612.09375v2 (26 August 2025), Theorem6.2.5, pp148–150; source copyright Tom Leinster 2014, 2016. Source: https://arxiv.org/abs/1612.09375v2 . AI changes notation and expands functor/naturality checks; Sections2–5 add proofs and exercises. Preserve attribution, source/licence links, marked changes, noncommercial and share-alike terms, and the licence warranty disclaimer. No endorsement is implied. Earlier course lessons retain their own licences.

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