Lemma 18.33.6. Let $\mathcal{C}$ be a site. Let $\mathcal{O}_1 \to \mathcal{O}_2$ be a homomorphism of presheaves of rings. Let $\mathcal{F}$ be a presheaf of $\mathcal{O}_2$-modules. There is a canonical short exact sequence

functorial in $\mathcal{F}$ called the *sequence of principal parts*.

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