$\Newextarrow{\xRightarrow}{5,5}{0x21D2}$ $\newcommand\empty{}$

Corollary Let $q: X \rightarrow S$ be an inner fibration of simplicial sets, let $z$ be a vertex of $X$, and let $e: s \rightarrow q(z)$ be an edge of $S$. Suppose that there exists a $q$-cartesian edge $g: y \rightarrow z$ of $X$ satisfying $q(g) = e$. Then any locally $q$-cartesian edge $h: x \rightarrow z$ satisfying $q(h) = e$ is $q$-cartesian.

Proof. By virtue of Remark, we may assume without loss of generality that $S$ is an $\infty $-category (or even a simplex). Applying Remark, we deduce that there is a $2$-simplex of $X$ as depicted in the diagram

\[ \xymatrix@R =50pt@C=50pt{ & y \ar [dr]^{g} & \\ x \ar [ur]^{f} \ar [rr]^-{h} & & z, } \]

where $f$ is an isomorphism in the $\infty $-category $X$. Then $f$ is also $q$-cartesian (Proposition, so Corollary guarantees that $h$ is $q$-cartesian. $\square$