Exercise 1: Interleaving Publishing and Consuming — Possible Solution
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THE SEQUENCE
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    q.publish('order-0'); q.publish('order-1'); q.publish('order-2')

    first = q.messages.pop(0)   # consume just 1, manually
    h(first)

    q.publish('order-3'); q.publish('order-4')

    q.consume_all(h)            # consume the rest

RESULTS AT EACH STEP
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    after consuming 1: ['order-0'] | remaining queued: ['order-1', 'order-2']
    queue after publishing 2 more: ['order-1', 'order-2', 'order-3', 'order-4']
    final processed order: ['order-0', 'order-1', 'order-2', 'order-3', 'order-4']

Consuming one message correctly processed order-0 and left order-1 and
order-2 still queued. Publishing two more messages while the queue
already had unconsumed items correctly appended them to the END of the
existing queue, not inserted anywhere else. The final consume_all()
call correctly processed the remaining four messages in the exact
order they were published: order-1, order-2 (published first, before
the pause), then order-3, order-4 (published after).

WHY THIS CONFIRMS GENUINE FIFO ORDERING ACROSS INTERLEAVED OPERATIONS
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This chapter's own original example published all 5 messages before
any consumption happened at all. This exercise interleaves publishing
and consuming - proving the queue's own ordering guarantee (first
published, first processed) holds even when production and
consumption happen in an interleaved, unpredictable sequence, not just
in the clean "publish everything, then consume everything" case. This
matters because a real system's producer and consumer genuinely run
independently, and this exercise confirms the queue's own ordering
doesn't depend on them running in any particular relative rhythm.

WHY THIS WORKS AS AN ANSWER
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The sequence directly interleaves publish and consume calls using this
chapter's own MessageQueue unmodified, and the queue's own contents and
the final processed order are checked at each individual step rather
than only at the end, confirming the ordering guarantee holds
throughout, not just in the final result.