Lemma 90.28.2. In the category $\widehat{\mathcal{C}}_\Lambda $, if $f_1, f_2 : R \to S$ are formally homotopic and $g : S \to S'$ is a morphism, then $g \circ f_1$ and $g \circ f_2$ are formally homotopic.
Proof. Namely, if $(r, h, k)$ is a formal homotopy between $f_1$ and $f_2$, then $(r, h, g \circ k)$ is a formal homotopy between $g \circ f_1$ and $g \circ f_2$. $\square$
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