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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def decode(f):
 """Extract a `MqttFixedHeader` from ``f``. Parameters f: file Object with read method. Raises ------- DecodeError When bytes decoded have values incompatible with a `MqttFixedHeader` object. UnderflowDecodeError When end-of-stream is encountered before the end of the fixed header. Returns ------- int Number of bytes consumed from ``f``. MqttFixedHeader Header object extracted from ``f``. """

        decoder = mqtt_io.FileDecoder(f)
        (byte_0,) = decoder.unpack(mqtt_io.FIELD_U8)

        packet_type_u4 = (byte_0 >> 4)
        flags = byte_0 & 0x0f

        try:
            packet_type = MqttControlPacketType(packet_type_u4)
        except ValueError:
            raise DecodeError('Unknown packet type 0x{:02x}.'.format(packet_type_u4))

        if not are_flags_valid(packet_type, flags):
            raise DecodeError('Invalid flags for packet type.')

        num_bytes, num_remaining_bytes = decoder.unpack_varint(4)

        return decoder.num_bytes_consumed, MqttFixedHeader(packet_type, flags, num_remaining_bytes)