I offer the following GPX extension as a starting point for a
standard. It's a work in progress. I've recently changed it to be
more compatible with geocaching.com.au. I expect it'll be changing
more. Might as well evolve it in a direction other sites think they
can use, too.

schema: http://geocaching.gpsgames.org/GPX/gpsgames.xsd
example: http://geocaching.gpsgames.org/GPX/gpsgames.gpx

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