Exercise 2: The Removed Admin/AJAX Guard — Possible Solution
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WHAT THE GUARD WAS ACTUALLY DOING
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Per this chapter, the is_admin()/DOING_AJAX check exists because
woocommerce_before_calculate_totals "also fires on ordinary wp-admin
page loads" - not only on the actual customer-facing cart/checkout.

THE PROBLEM WITH REMOVING IT
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Per this chapter, without the guard, "this same code could run in
contexts with no real cart to discount, or interfere with the admin
order-editing screen." Since the hook fires in the admin area too,
the discount logic would execute in situations it was never intended
for - potentially running against no meaningful cart data, or
unexpectedly altering prices while an admin is manually editing an
order in wp-admin.

WHY THIS ISN'T JUST BOILERPLATE
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Per this chapter, this guard is described as "a small, real, commonly
-forgotten detail specific to this exact hook" - it's not a generic
defensive habit applied everywhere, but a fix for a real, documented
quirk of this one particular hook firing in more places than a
developer might initially assume.

WHY THIS WORKS AS AN ANSWER
------------------------------
It correctly identifies why the hook needs the guard (it fires in
admin contexts too), correctly describes the concrete problem removing
it causes, and correctly frames the guard as addressing a real,
hook-specific quirk rather than generic unnecessary boilerplate.