%%
%% The LaTeX Companion, 3ed
%%
%% Example A-1-15 on page II-641 in "Using the body of an environment as an argument".
%%
%% 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{111.0pt}

  \addtolength\textwidth{1pt}

%StartShownPreambleCommands
\NewDocumentEnvironment{twice}{O{\ttfamily} +b}{#2#1#2?}{*}
%StopShownPreambleCommands

\begin{document}
\begin{twice}[\itshape] Hello world: \par Lighten up! \end{twice}
\par \begin{twice} T \end{twice}\begin{twice} wice \end{twice}
\end{document}