%%
%% The LaTeX Companion, 3ed
%%
%% Example A-1-11 on page II-636 in "Creating document commands and 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{159.0pt}
%StartShownPreambleCommands
\NewExpandableDocumentCommand\header{O{1}m}
       {\multicolumn{#1}{c}{\bfseries #2}}
%StopShownPreambleCommands

\begin{document}
\begin{tabular}{lll}
  \header[2]{2-cols} & \header{1-col} \\
   --  & --- &  \\ `! & `? & f ff fi ffi fl ffl
\end{tabular}
\end{document}