*     not contain null or [Protocol.HTTP_1_0].
     */
    fun protocols(protocols: List<Protocol>) =
      apply {
        // Create a private copy of the list.
        val protocolsCopy = protocols.toMutableList()

        // Validate that the list has everything we require and nothing we forbid.
        require(Protocol.H2_PRIOR_KNOWLEDGE in protocolsCopy || HTTP_1_1 in protocolsCopy) {