# What package(s) do I need to use to work \pgfdeclareshape and \pgf@xa, \pgf@xb, pgf@xc?

I have been trying to use the following example from the TikZ and PGF Manual (ver. 3.1.3) pg. 1126, but when I do I get multiple errors. In particular, the compiler doesn't seem to recognize the names \pgf@xa, \pgf@xb, and \pgf@xc. It also states that the @ shouldn't come after \advance. I have tried various \usepackage{} statements but with no luck.And I am not clear if the \pgfdeclareshape should be within the scope of the document or before it.

\pgfdeclareshape{document}{
\inheritsavedanchors[from=rectangle] % this is nearly a rectangle
\inheritanchorborder[from=rectangle]
\inheritanchor[from=rectangle]{center}
\inheritanchor[from=rectangle]{north}
\inheritanchor[from=rectangle]{south}
\inheritanchor[from=rectangle]{west}
\inheritanchor[from=rectangle]{east}
% ... and possibly more
\backgroundpath{% this is new
% store lower right in xa/ya and upper right in xb/yb
\southwest \pgf@xa=\pgf@x \pgf@ya=\pgf@y
\northeast \pgf@xb=\pgf@x \pgf@yb=\pgf@y
% compute corner of ‘‘flipped page’’
\pgf@xc=\pgf@xb \advance\pgf@xc by-5pt % this should be a parameter
% construct main path
\pgfpathmoveto{\pgfpoint{\pgf@xa}{\pgf@ya}}
\pgfpathlineto{\pgfpoint{\pgf@xa}{\pgf@yb}}
\pgfpathlineto{\pgfpoint{\pgf@xc}{\pgf@yb}}
\pgfpathlineto{\pgfpoint{\pgf@xb}{\pgf@yc}}
\pgfpathlineto{\pgfpoint{\pgf@xb}{\pgf@ya}}
\pgfpathclose
% add little corner
\pgfpathmoveto{\pgfpoint{\pgf@xc}{\pgf@yb}}
\pgfpathlineto{\pgfpoint{\pgf@xc}{\pgf@yc}}
\pgfpathlineto{\pgfpoint{\pgf@xb}{\pgf@yc}}
\pgfpathlineto{\pgfpoint{\pgf@xc}{\pgf@yc}}
}
}\hskip-1.2cm
\begin{tikzpicture}
\node[shade,draw,shape=document,inner sep=2ex] (x) {Remark};
\node[fill=yellow!80!black,draw,ellipse,double]
at ([shift=(-80:3cm)]x) (y) {Use Case};
\draw[dashed] (x) -- (y);
\end{tikzpicture}


Welcome to TeX-SE! In LaTeX @ plays a special role, it is a nonstandard character unless you are in a package, or you use \makeatletter. This is to protect certain macros, dimensions etc. from accidentally being overwritten. So you may want to wrap the declaration of the shape in \makeatletter ... \makeatother, where the latter switches back to normal. In your example you also need shapes.geometric for the ellipse.

\documentclass[tikz,border=3.14mm]{standalone}
\usetikzlibrary{shapes.geometric}
\begin{document}
\makeatletter
\pgfdeclareshape{document}{
\inheritsavedanchors[from=rectangle] % this is nearly a rectangle
\inheritanchorborder[from=rectangle]
\inheritanchor[from=rectangle]{center}
\inheritanchor[from=rectangle]{north}
\inheritanchor[from=rectangle]{south}
\inheritanchor[from=rectangle]{west}
\inheritanchor[from=rectangle]{east}
% ... and possibly more
\backgroundpath{% this is new
% store lower right in xa/ya and upper right in xb/yb
\southwest \pgf@xa=\pgf@x \pgf@ya=\pgf@y
\northeast \pgf@xb=\pgf@x \pgf@yb=\pgf@y
% compute corner of ''flipped page''
\pgf@xc=\pgf@xb \advance\pgf@xc by-5pt % this should be a parameter
% construct main path
\pgfpathmoveto{\pgfpoint{\pgf@xa}{\pgf@ya}}
\pgfpathlineto{\pgfpoint{\pgf@xa}{\pgf@yb}}
\pgfpathlineto{\pgfpoint{\pgf@xc}{\pgf@yb}}
\pgfpathlineto{\pgfpoint{\pgf@xb}{\pgf@yc}}
\pgfpathlineto{\pgfpoint{\pgf@xb}{\pgf@ya}}
\pgfpathclose
% add little corner
\pgfpathmoveto{\pgfpoint{\pgf@xc}{\pgf@yb}}
\pgfpathlineto{\pgfpoint{\pgf@xc}{\pgf@yc}}
\pgfpathlineto{\pgfpoint{\pgf@xb}{\pgf@yc}}
\pgfpathlineto{\pgfpoint{\pgf@xc}{\pgf@yc}}
}
}
\makeatother
\begin{tikzpicture}
\node[shade,draw,shape=document,inner sep=2ex] (x) {Remark};
\node[fill=yellow!80!black,draw,ellipse,double]
at ([shift=(-80:3cm)]x) (y) {Use Case};
\draw[dashed] (x) -- (y);
\end{tikzpicture}
\end{document}