Sippet.Message.put_new_header

You're seeing just the function put_new_header, go back to Sippet.Message module for more information.
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put_new_header(message, header, value)

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Specs

put_new_header(t(), header(), value()) :: t()

Puts the value under header on the message unless the header already exists.

Examples:

iex> Sippet.Message.build_request(:invite, "sip:foo@bar.com")
...> |> Sippet.Message.put_new_header(:max_forwards, 70)
...> |> Sippet.Message.put_new_header(:max_forwards, 1)
%Sippet.Message{body: nil, headers: %{max_forwards: 70},
 start_line: %Sippet.Message.RequestLine{method: :invite,
  request_uri: %Sippet.URI{authority: "foo@bar.com", headers: nil,
   host: "bar.com", parameters: nil, port: 5060, scheme: "sip",
   userinfo: "foo"}, version: {2, 0}}, target: nil}