respond_to for unknown type

Greetings,

How would I go about handling an unknown type in a respond_to block?

That is, I currently have ...

respond_to do | format |
  format.html { do something }
  format.xml { do something else }
end

I would like a handler for all other format's, other than html/xml, so
that I can, for example, log the request, render an error or whatnot.

Thank you,

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