# File lib/heroku/client.rb, line 546
  def extract_warning(response)
    return unless response
    if response.headers[:x_heroku_warning] && @warning_callback
      warning = response.headers[:x_heroku_warning]
      @displayed_warnings ||= {}
      unless @displayed_warnings[warning]
        @warning_callback.call(warning)
        @displayed_warnings[warning] = true
      end
    end
  end