is_valid(packet) -> indicates if the packet's contents conform to the SLIP specification.
    
    A packet is valid if:

    * It contains no :const:`END` bytes other than leading and/or trailing :const:`END` bytes, and
    * Each :const:`ESC` byte is followed by either an :const:`ESC_END` or an :const:`ESC_ESC` byte.

    :param bytes packet: The packet to inspect.
    :return: :const:`True` if the packet is valid, :const:`False` otherwise
    :rtype: bool

def is_valid(packet):
    """is_valid(packet) -> indicates if the packet's contents conform to the SLIP specification.
    
    A packet is valid if:

    * It contains no :const:`END` bytes other than leading and/or trailing :const:`END` bytes, and
    * Each :const:`ESC` byte is followed by either an :const:`ESC_END` or an :const:`ESC_ESC` byte.

    :param bytes packet: The packet to inspect.
    :return: :const:`True` if the packet is valid, :const:`False` otherwise
    :rtype: bool
    """
    packet = packet.strip(END)
    return not (END in packet or
                packet.endswith(ESC) or
                re.search(ESC + b'[^' + ESC_END + ESC_ESC + b']', packet))