%%
%% The LaTeX Companion, 3ed
%%
%% Example 8-3-8 on page I-606 in "Extended or changed commands".
%%
%% 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}

%StartShownPreambleCommands
\usepackage{xcolor,pict2e}
\NewDocumentCommand\ShowControls{r()d()}
   {\put(#1){\circle*{3}}\IfNoValueF{#2}{\put(#2){\circle{3}}}}
%StopShownPreambleCommands

\begin{document}
\begin{picture}(110,90)
 \put(0,0){\framebox(110,90){}}
 \qbezier[150](5,5)(20,80)(35,5)     \ShowControls(20,80)
 \qbezier[50](5,5)(70,85)(105,85)    \ShowControls(70,85)
\color{blue}
 \qbezier[0](5,5)(40,80)(35,5)       \ShowControls(40,80)
 \cbezier(70,5)(60,40)(90,40)(105,85)\ShowControls(60,40)(90,40)
\end{picture}
\end{document}