google sitemaps and custom tags

I would li开发者_如何学JAVAke to add a custom tag or attribute to the google sitemap for internal use. I can't find any documentation on this so I would like to ask if anybody has done something similar?

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You can't add your own tags to an XML sitemap as they are not valid as described in the schema. If you do this you run the risk of the sitemap being rejected or causing errors.

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I am using custom attributes (seem to work fine) and elements (cause errors, but are ignored by Google Webmaster Tools).

See details here: https://stackoverflow.com/a/32628830/1117929