 
   If the parameter `value` is `nil`, then the empty list will be appended to
   the `header`.
+
+  ## Examples:
+
+      iex> request = Sippet.Message.build_request :invite, "sip:foo@bar.com"
+      iex> request = Sippet.Message.put_header_back(request, :content_language,
+      ...>   "en-US")
+      iex> request |> Sippet.Message.put_header_back(:content_language,
+      ...>   "de-DE")
+      %Sippet.Message{body: nil, headers: %{content_language: ["en-US", "de-DE"]},
+       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}
+
   """
   @spec put_header_back(t, header, multiple_value) :: t
   def put_header_back(message, header, value) do


  @doc """
  Deletes the last value of `header` in `message`.
  """
  @spec delete_header_back(t, header) :: t
  def delete_header_back(message, header) do


  If the parameter `value` is `nil`, then the empty list will be prefixed to
  the `header`.

  ## Examples:

      iex> request = Sippet.Message.build_request :invite, "sip:foo@bar.com"
      iex> request = Sippet.Message.put_header_front(request, :content_language,
      ...>   "de-DE")
      iex> request |> Sippet.Message.put_header_front(:content_language,
      ...>   "en-US")
      %Sippet.Message{body: nil, headers: %{content_language: ["en-US", "de-DE"]},
       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}

  """
  @spec put_header_front(t, header, multiple_value) :: t
  def put_header_front(message, header, value) do
