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Corollary 4.3.7.17. Let $q: X \rightarrow S$ be a trivial Kan fibration of simplicial sets and let $f: K \rightarrow X$ be any morphism of simplicial sets. Then the induced maps

\[ X_{/f} \rightarrow S_{/ (q \circ f)} \quad \quad X_{f/} \rightarrow S_{(q \circ f)/} \]

are trivial Kan fibrations.