%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-4-18 on page II-171 in "diffcoeff --- Handling derivatives of arbitrary order".
%%
%% 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{tlc3examargin}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{171.0pt}

  % Shorten the vertical size of the example ...
  \AtBeginDocument{%
    \setlength\abovedisplayskip{.4\abovedisplayskip}%
    \setlength\belowdisplayskip{.4\belowdisplayskip}%
  }

%StartShownPreambleCommands
\usepackage[ISO]{diffcoeff}
%StopShownPreambleCommands

\begin{document}
\[ \dl P = \diffp Px \dl x + \diffp Py \dl y
           + \diffp Pz \dl z                  \]
\end{document}