To answer this reddit question https://www.reddit.com/r/elixir/comments/4qetef/pattern_matching_with_a_list_of_tuples/

cookie.ex

defmodule Cookie do
  def run([ { "Set-Cookie", _} | [{"Set-Cookie", val} | _] ]), do: val
  def run([ _ | rest ]), do: run(rest)
end

cookie_test.exs

defmodule CookieTest do
  use ExUnit.Case
  test "find second cookie val" do
    resp = [{"Server", "nginx"}, {"Date", "Thu, 30 Jun 2016 07:23:20 GMT"},
      {"Content-Type", "application/xml;charset=UTF-8"}, {"Content-Length", "17"},
      {"Connection", "keep-alive"},
      {"Set-Cookie", "OTHERCOOKIE;Path=/youtrack;HttpOnly"},
      {"Set-Cookie",
        "jetbrains.charisma.main.security.PRINCIPAL=COOKIE;Path=/youtrack;Expires=Fri, 30-Jun-2017 07:23:20 GMT"},
      {"Expires", "Thu, 01 Jan 1970 00:00:00 GMT"},
      {"Access-Control-Expose-Headers", "Location"},
      {"Cache-Control", "no-cache, no-store, no-transform, must-revalidate"},
      {"X-Content-Type-Options", "nosniff"}
    ]
    assert Cookie.run(resp) ===  "jetbrains.charisma.main.security.PRINCIPAL=COOKIE;Path=/youtrack;Expires=Fri, 30-Jun-2017 07:23:20 GMT"
  end
end