Challenge 3: Why Collections Need add/remove/clear -- Solution
Walkthrough

A scalar setting -- a boolean, a single numeric limit -- has exactly
one value at any given point in the hierarchy, so "the deepest
web.config wins" is unambiguous: there's nothing to combine, only one
value to pick. A collection (a list of default documents, a list of
MIME types, a list of installed modules) is different, because in
most real cases a subdirectory doesn't want to throw away everything
the site root or machine level already configured -- it wants to add
one or two extra entries on top of what's already there, or remove
one specific inherited entry, while leaving the rest intact.

If IIS simply let a deeper web.config's collection value silently
replace a shallower one wholesale, there would be no way to express
"keep everything already inherited, plus this one new entry" --
every web.config that wanted to add even a single item would have to
fully re-list every item from every level above it, which is fragile
(a future change to the parent list wouldn't propagate down) and
easy to get wrong. The explicit add/remove/clear actions solve this:
add appends to what's inherited, remove takes out one named item
without disturbing the rest, and clear is the deliberate escape
hatch for the cases that do want to discard the inherited list
entirely and start fresh.

WHY THIS WORKS AS AN ANSWER
------------------------------
This exercise checks that the reader understands why IIS's
configuration schema treats collections as a genuinely different
kind of setting from scalars, rather than just memorizing that
add/remove/clear exist -- the underlying reason is that inheritance
by default should be additive for a list, not overwritten with no
way to preserve or selectively remove what came before.