problem renaming command in SIunits

Question

I've developed a package in which I want to define a command \power, but that name is taken by a command in the SIunits package (which my package uses). I initially named my command \powr but decided to try renaming the \power command in SIunits and use \power in my own package. The problem is that I've apparently missed something or done something incorrect because I can't get the intended result.

(Note that I don't want to use siunitx over SIunits at present, although I will migrate to it in the future.)

Here is a MWE which does NOT included all the packages my own package uses. My major problem is that \square doesn't work as after renaming \power and I simply don't understand why. \sisquare doesn't work either but I don't use it. What am I missing?

\documentclass{article}
\usepackage{letltxmacro}
\usepackage{SIunits}

\newcommand*{\m}{\ensuremath{\mathrm{m}}

\LetLtxMacro{\sipower}{\power}
\renewcommand{\power}[1]{\ensuremath{#1 \; \mathrm{J/s}}}

\LetLtxMacro{\sisquare}{\square}
\renewcommand*{\square}[1]{\mathrm{#1}^{2}}

\renewcommand*{\squared}{\ensuremath{^{\mathrm{2}}}}
\renewcommand*{\cubic}[1]{\sipower{#1}{3}}
\renewcommand*{\cubed}{\ensuremath{^{\mathrm{3}}}}
\renewcommand*{\fourth}[1]{\sipower{#1}{4}}
\renewcommand*{\reciprocal}[1]{\sipower{#1}{\SIminus1}}
\renewcommand*{\rp}{\ensuremath{^{\mathrm{\SIminus1}}}}
\renewcommand*{\rpsquare}[1]{\sipower{#1}{\SIminus2}}
\renewcommand*{\rpsquared}{\ensuremath{^{\mathrm{\SIminus2}}}}
\renewcommand*{\rpcubic}[1]{\sipower{#1}{\SIminus3}}
\renewcommand*{\rpcubed}{\ensuremath{^{\mathrm{\SIminus3}}}}
\renewcommand*{\rpfourth}[1]{\sipower{#1}{\SIminus4}}

\begin{document}

\sipower{\m}{2} \par
\power{3}       \par
$\sisquare\m$   \par
\square\m       \par
\m\squared      \par
\cubic\m        \par
\m\cubed        \par
\fourth\m       \par
\reciprocal\m   \par
\m\rp           \par
\rpsquare\m     \par
\m\rpsquared    \par
\rpcubic\m      \par
\m\rpcubed      \par
\rpfourth\m

\end{document}

Answer 1

The SIunits package uses the \AtBeginDocument hook to (re)define \square as that macro name is also used by the amssymb package for a (graphical) square.

Therefore it provides two package options that deal with this clash:

• amssymb overwrites \square with no backup of amssymb’s original \square macro.
• squaren provides the macro name \squaren instead and leaves \square alone.

As the user might load amssymb after SIunits this redefiniton problem is postponed until \begin{document} (after that you can’t load any packages):

%%%%%% siunits.sty, lines 75ff.
%%%%%% \typeout messages removed
%%%%%% Comments provided by me           \%/

\AtBeginDocument{%
 \ifredefsquare                           % option "amssymb": \square is overwritten
  \providecommand{\square}[1]{\power{#1}{2}}
  \renewcommand{\square}[1]{\power{#1}{2}}
 \else
   \ifdefsquaren                          % option "squaren": \squaren is defined
    \providecommand{\squaren}[1]{\power{#1}{2}}
    \renewcommand{\squaren}[1]{\power{#1}{2}}
   \else                                  % neither "amssymb" nor "squaren"
    \@ifundefined{square}{%               % if \square is undefined, define it (no amssymb)
     \newcommand*{\square}[1]{\power{#1}{2}}
     }{%                                  % if \square is defined, yell at user:
     \PackageWarningNoLine{SIunits}{%     % "Use either amssymb or squaren option"
     The command `square' was already defined.\MessageBreak
     Possibly due to the amssymb package}
    }  %\ifundefined{square}
   \fi %\ifsefsquaren
 \fi   %\ifredefsquare
 }     %\AtBeginDocument

The solution to your problem is to hook yourself into \AtBeginDocument and redefine \square there.

I also patched \sisquare as it is internally defined with \power that you also redefine (similar reasons why you need to redefine all those other macros with \sipower).

Code

\documentclass{article}
\usepackage{letltxmacro}
\usepackage{SIunits}
\newcommand*{\m}{\ensuremath{\mathrm{m}}}

\renewcommand*{\squared}{\ensuremath{^{\mathrm{2}}}}
\renewcommand*{\cubed}{\ensuremath{^{\mathrm{3}}}}
\renewcommand*{\cubic}[1]{\sipower{#1}{3}}
\renewcommand*{\fourth}[1]{\sipower{#1}{4}}

\renewcommand*{\reciprocal}[1]{\sipower{#1}{\SIminus1}}
\renewcommand*{\rpsquare}[1]{\sipower{#1}{\SIminus2}}
\renewcommand*{\rpcubic}[1]{\sipower{#1}{\SIminus3}}
\renewcommand*{\rpfourth}[1]{\sipower{#1}{\SIminus4}}

\renewcommand*{\rp}{\ensuremath{^{\mathrm{\SIminus1}}}}
\renewcommand*{\rpsquared}{\ensuremath{^{\mathrm{\SIminus2}}}}
\renewcommand*{\rpcubed}{\ensuremath{^{\mathrm{\SIminus3}}}}

\LetLtxMacro{\sipower}{\power}%
\renewcommand*{\power}[1]{\ensuremath{#1 \; \mathrm{J/s}}}%

\AtBeginDocument{%
    \LetLtxMacro{\sisquare}{\square}%
    \renewcommand*{\square}[1]{\mathrm{#1}^{2}}%
    \renewcommand*{\sisquare}[1]{\sipower{#1}{2}}% to correct the output of the original square, now sisquare, command
}

\begin{document}
\sipower{\m}{5} \par
$\sisquare\m$   \par
\power{120}     \par
$\square\m$     \par\bigskip

\m\squared      \par
\cubic\m        \par
\m\cubed        \par
\fourth\m       \par
\reciprocal\m   \par
\m\rp           \par
\rpsquare\m     \par
\m\rpsquared    \par
\rpcubic\m      \par
\m\rpcubed      \par
\rpfourth\m
\end{document}

Output