\section{Acknowledgements}
\label{sec:ack}

The document reflects the discussion on the SIMPLE mailing list, with
contributions from many individuals.  Markus Isomaki, Hisham Khartabil,
Jon Peterson and Brian Rosen provided detailed comments and suggestions. 
The notion of external references in the \xmlelement{card},
\xmlelement{icon} and \xmlelement{info} elements is an evolution of the
BINPIDF proposal by Khartabil et al.
