\documentclass{article}

\input{../../preamble}

\newcommand{\link}[1]{\lean{../..}
  {Bookshelf/Enderton/Chapter_0} % Location
  {Enderton.Chapter_0.#1} % Namespace
  {Chapter_0.#1} % Presentation
}

\begin{document}

\header{Useful Facts About Sets}{Herbert B. Enderton}

\section{\proceeding{Lemma 0A}}%
\hyperlabel{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}