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When there are multiple versions of a document, it is sometimes useful to be able to ask for the complete list of available variants. This may be needed if none of the preferred languages matches the available ones, or in the context of language engineering, when one is comparing versions.
HTTP 1.1 proposes a `300 Multiple Choices' error return code, that a server can send to a client, along with a description of the available choices (presumably in HTML).
It also proposes headers `Vary' and `Alternates' that a server can send along with a successful response, to tell the client about the existence of alternate versions.