Wie kann ich zu einem shape zusätzliche Anker definieren?

\documentclass[tikz,margin=5mm,convert=false]{standalone}
\makeatletter
\pgfdeclareshape{myrectangle}{
  % es wird zunächst alles vom rectangle übernommen
  \inheritsavedanchors[from=rectangle]
  \inheritanchorborder[from=rectangle]
  \foreach \a in {%
      center,mid,base,north,south,west,east,%
      north west,mid west,base west,south west,%
      north east,mid east,base east,south east%
    }{\inheritanchor[from=rectangle]{\a}}
  \inheritbackgroundpath[from=rectangle]
  % dann werden die zusätzlichen Anker ergänzt
  \anchor{north 1/3}{
    \southwest\pgf@xa=\pgf@x
    \northeast\pgfmathsetlength\pgf@x{\pgf@xa-(\pgf@xa-\pgf@x)/3}
  }
  \anchor{north 2/3}{
    \southwest\pgf@xa=\pgf@x
    \northeast\pgfmathsetlength\pgf@x{\pgf@x-(\pgf@x-\pgf@xa)/3}
  }
  \anchor{south 1/3}{
    \northeast\pgf@xa=\pgf@x
    \southwest\pgfmathsetlength\pgf@x{\pgf@x-(\pgf@x-\pgf@xa)/3}
  }
  \anchor{south 2/3}{
    \northeast\pgf@xa=\pgf@x
    \southwest\pgfmathsetlength\pgf@x{\pgf@xa-(\pgf@xa-\pgf@x)/3}
  }
  \anchor{east 1/3}{
    \southwest\pgf@ya=\pgf@y
    \northeast\pgfmathsetlength\pgf@y{\pgf@ya-(\pgf@ya-\pgf@y)/3}
  }
  \anchor{east 2/3}{
    \southwest\pgf@ya=\pgf@y
    \northeast\pgfmathsetlength\pgf@y{\pgf@y-(\pgf@y-\pgf@ya)/3}
  }
  \anchor{west 1/3}{
    \northeast\pgf@ya=\pgf@y
    \southwest\pgfmathsetlength\pgf@y{\pgf@y-(\pgf@y-\pgf@ya)/3}
  }
  \anchor{west 2/3}{
    \northeast\pgf@ya=\pgf@y
    \southwest\pgfmathsetlength\pgf@y{\pgf@ya-(\pgf@ya-\pgf@y)/3}
  }
}
\makeatother

\tikzset{
  shape example/.style= {color = black!30,draw,fill = yellow!30,
    line width = .5cm,inner xsep = 2.5cm,inner ysep = 0.5cm}
}

\begin{document}
\Huge
\begin{tikzpicture}
  \node[name=s,shape=myrectangle,shape example] {Rectangle\vrule width 1pt height 2cm};
  % bisherige Anker einzeichnen lassen
  \foreach \anchor/\placement in
    {north west/above left, north/above, north east/above right,
      west/left, center/above, east/right,
      mid west/right, mid/above, mid east/left,
      base west/left, base/below, base east/right,
      south west/below left, south/below, south east/below right,
      text/left, 15/right,150/above}
    \draw[shift=(s.\anchor)] plot[mark=x] coordinates{(0,0)}
      node[\placement,font=\scriptsize] {\texttt{(s.\anchor)}};
  % neue Anker einzeichnen lassen
  \foreach \anchor/\placement in
    {{north 1/3}/above,{north 2/3}/above,{south 1/3}/below,{south 2/3}/below,
      {east 1/3}/right,{east 2/3}/right,{west 1/3}/left,{west 2/3}/left}
    \draw[shift=(s.\anchor),red] plot[mark=x] coordinates{(0,0)}
      node[\placement,font=\scriptsize\bfseries] {\texttt{(s.\anchor)}};
\end{tikzpicture}
\end{document}