\begin{xy}
% \xyWARMprocessMo{\Fname}{}
 \xyShowAllMarkedPoints{}{\Fname}{\grExt}%
 \xyMarkedImport{\Fname}% repeat name else pdfTeX crashes
 \xyMarkedPos{pn}*++!L\txt{force across $S$\\is $-p\mathbf{n}$}
 \xyMarkedPos{n}*++!L{\mathbf{n}}
 \xyMarkedPos{S}*+!LD{S}
\end{xy}