%%
%% The LaTeX Companion, 3ed
%%
%% Example 4-3-29 on page I-355 in "Actions based on the current column".
%%
%% 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{243.0pt}
\setlength\textheight{12\baselineskip}
% small headings for the example
\makeatletter
\renewcommand\section{\@startsection{section}{1}{\z@}%
{-1.25ex \@plus-1ex \@minus-.2ex}%
{.5ex}%
{\normalfont\large\bfseries}}
\makeatother
%StartShownPreambleCommands
\usepackage{lipsum,ragged2e}
\usepackage[colaction]{multicol}
\newcommand\lcolsection{\par
\docolaction*{}{}{\columnbreak}%
\section}
%StopShownPreambleCommands
\begin{document}
\begin{multicols}{2} \RaggedRight
\lcolsection{A} \lipsum[1][1-2]
\lcolsection{B} \lipsum[2][1-4]
\lcolsection{C} \lipsum[3]
\end{multicols}
\end{document}
