Lemma 3 (sign clause): $f\circ T=\pm f$ for every $T\in\mathrm{Sym}(C)$, and $f\circ T=f$ if $T$ reverses orientation

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Lemma 3 (sign clause): $f\circ T=\pm f$ for every $T\in\mathrm{Sym}(C)$, and $f\circ T=f$ if $T$ reverses orientation · Prove2Me