code: def to_s
        super + ":\n" +
        format_observation(result.control) + "\n" +
        result.candidates.map { |candidate| format_observation(candidate) }.join("\n") +
        "\n"
      end
docs: The default formatting is nearly unreadable, so make it useful.
  
   The assumption here is that errors raised in a test environment are
   printed out as strings, rather than using #inspect.