\documentclass{article}

\input{preamble}

\newcommand{\link}[1]{\lean{../..}
  {Exercises/Enderton/Chapter0}
  {Exercises.Enderton.Chapter0.#1}
  {Chapter0.#1}
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\begin{document}

\section*{Lemma 0A}%
\label{sec:lemma-0a}

Assume that $\langle x_1, \ldots, x_m \rangle = \langle y_1, \ldots, y_m, \ldots, y_{m+k} \rangle$.
Then $x_1 = \langle y_1, \ldots, y_{k+1} \rangle$.

\begin{proof}

  \link{lemma\_0a}

\end{proof}

\end{document}