Example of a homeomorphism between a doughnut and a coffee cup
discipline was immature.
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The honest answer: because it shouldn’t work and has never been done before. TeX has no arrays, no functions with return values, no convenient local variables or stack frames, no integers bigger than 2^{31}-1, no bitwise operations. Macro expansion can recurse, but you get no call stack and deep recursion quickly hits engine limits. What TeX does have is a Turing-complete macro expansion engine and, with e-TeX extensions (used by modern pdfTeX), up to 32,768 integer registers called \count. That turns out to be just barely enough to implement a chess engine.
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