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+\documentclass{article}
+\usepackage{amsfonts, amsthm}
+\usepackage{hyperref}
+
+\newtheorem{theorem}{Theorem}
+
+\begin{document}
+
+\begin{theorem}[Lemma 0A]
+
+Assume that $\langle x_1, \ldots, x_m \rangle = \langle y_1, \ldots, y_m, \ldots, y_{m+k} \rangle$.
+Then $x_1 = \langle y_1, \ldots, y_{k+1} \rangle$.
+
+\end{theorem}
+
+\begin{proof}
+
+\href{Chapter0.lean}{Enderton.Chapter0.lemma\_0a}
+
+\end{proof}
+
+\end{document}