LaTeX-kode hentet fra nettet

Jeg har fundet en LaTeX-kodestump, der skulle lave en plimsoll, det kemiske symbol for standardtilstanden (en cirkel med en horisontal streg igennem, http://en.wikipedia.org/wiki/Plimsol_line ).
Koden fandt jeg på
http://groups.google.com/group/comp.text.tex/browse_thread/thread/9ccd5c1acb4310de/11c409fdc881abf8?lnk=st&q=%5Cmoverlay&rnum=3&hl=en#11c409fdc881abf8 .

Mit dokument er
\documentclass{article}

\def\moverlay{\mathpalette\mov@rlay}
\def\mov@rlay#1#2{\leavevmode\vtop{%
\baselineskip\z@skip \lineskiplimit-\maxdimen
\ialign{\hfil$#1##$\hfil\cr#2\crcr}}}


ewcommand\barcirc{\protect\moverlay{\circ\cr-}}

\begin{document}
$\barcirc$
\end{document}

Så får jeg flg. fejlmeddelelse:
! Use of \mov doesn't match its definition.
\mathpalette #1#2->\mathchoice {#1\displaystyle
{#2}}{#1\textstyle {#2}}{#1\...
l.15 $\barcirc
$
?
! Use of \mov doesn't match its definition.
\mathpalette ...1\displaystyle {#2}}{#1\textstyle
{#2}}{#1\scriptstyle {#2}}...
l.15 $\barcirc
$
?
! Use of \mov doesn't match its definition.
\mathpalette ...#1\textstyle {#2}}{#1\scriptstyle
{#2}}{#1\scriptscriptstyle...
l.15 $\barcirc
$
?
! Use of \mov doesn't match its definition.
\mathpalette ...tstyle {#2}}{#1\scriptscriptstyle
{#2}}
l.15 $\barcirc
$
?
! Misplaced \cr.
\barcirc ->\protect \moverlay {\circ \cr
-}

Nogen, der ved hvorfor?