Sippet.Message.get_and_update

You're seeing just the function get_and_update, go back to Sippet.Message module for more information.
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get_and_update(message, key, fun)

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Gets the value from key and updates it, all in one pass.

About the same as Map.get_and_update/3 except that this function actually does not remove the key from the struct case the passed function returns :pop; it puts nil for :start_line, :body and :target ands %{} for the :headers key.