Marginal enumeration not in the margins

Question

Based on an answer to one of my previous questions, I implemented the following code which is supposed to enumerate examples discretely in the margins of my document:

\documentclass{memoir}

\usepackage[latin1]{inputenc}
\usepackage[T1]{fontenc}

\usepackage{amsmath,amssymb,geometry,graphicx,amstext}

\usepackage[amsmath]{ntheorem}

% margin theorem
\makeatletter
\newtheoremstyle{mymargin}%
{\item[\theorem@headerfont \llap{##1 ##2}]}%
{\item[\theorem@headerfont \llap{##1 ##2}| ##3\theorem@separator\hskip\labelsep]}%
\makeatother

% my example
\theoremstyle{mymargin}
\newtheorem{example}{Example}
\numberwithin{example}{chapter}
\begin{document}
    \begin{example}
        \centering
        \begin{tabular}{cccc}
            this & is & a & test\\
            table & where  & text & goes
        \end{tabular}
    \end{example}
\end{document}

This produces the following output:

As you see, because the table is centered, the example number is not actually in the margins, which is my major concern. Furthermore, I would like the number to appear aligned with the top of the table and always on the same page as the table (which is not always the case either). Finally, I would like the number always to appear in the outer margin when the two-page option is on. How may this be fixed?

Answer 1

\centering is a bit too much here and won't be appropriate if there is following text. Use \begin{center}...\end{center] escaped with a preceeding \leavevmode:

\documentclass{memoir}

\usepackage[latin1]{inputenc}
\usepackage[T1]{fontenc}

\usepackage{amsmath,amssymb,geometry,graphicx,amstext}

\usepackage[amsmath]{ntheorem}

% margin theorem
\makeatletter
\newtheoremstyle{mymargin}%
{\item[\theorem@headerfont \llap{##1 ##2}]}%
{\item[\theorem@headerfont \llap{##1 ##2}| ##3\theorem@separator\hskip\labelsep]}%
\makeatother

% my example
\theoremstyle{mymargin}
\newtheorem{example}{Example}
\numberwithin{example}{chapter}
\begin{document}

\begin{example}
  Ordinary example.
\end{example}

\begin{example}
\leavevmode\begin{center}
    \begin{tabular}{cccc}
      this & is & a & test\\
      table & where  & text & goes
    \end{tabular}
  \end{center}
  And now comes the discussion of the rest of the example.
\end{example}

\begin{example}
  \begin{equation*}
    x=y
  \end{equation*}
\end{example}

\end{document}