%%
%% The LaTeX Companion, 3ed
%%
%% Example 2-4-17 on page I-92 in "Handling theorem-like environments".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{171.0pt}
%StartShownPreambleCommands
\usepackage{cleveref}
\crefname{thm}{theorem}{theorems}
\crefname{lem}{lemma}{lemmas}
\newtheorem{thm}{Theorem} \newtheorem{lem}[thm]{Lemma}
%StopShownPreambleCommands

\begin{document}
\begin{thm} A theorem.  \label{thm:a}\end{thm}
\begin{lem} A lemma.    \label{lem:a}\end{lem}
\begin{lem} Another one.\label{lem:b}\end{lem}
\Cref{lem:b} is used to prove \cref{thm:a}.    \\
But \cref{lem:a,lem:b} need formatting help.
\end{document}