\documentclass{article}

\input{../../preamble}

\newcommand{\lean}[1]{\leanref
  {./Chapter\_0.html\#Enderton.Chapter\_0.#1}
  {Enderton.Chapter\_0.#1}}

\begin{document}

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

\section*{\proceeding{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}

  \lean{lemma\_0a}

\end{proof}

\end{document}