nextBlock += length;
        return pos;
    }

    private final class BlockImpl extends Block {
        private static final int HEADER_SIZE = 1 + INT_SIZE; // type, payload size
        private static final int TAIL_SIZE = INT_SIZE;

        private BlockPointer pos;
        private int payloadSize;

        private BlockImpl(BlockPayload payload, BlockPointer pos) {
            this(payload);