Sippet.Message.pop

You're seeing just the function pop, go back to Sippet.Message module for more information.
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pop(message, key, default \\ nil)

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Returns and removes the value associated with key in message.

About the same as Map.pop/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.