An `std::vector<T>::reverse_iterator` stores the
`std::vector<T>::iterator` which points to the (forwards-ordered)
*following* item. Thus while `vec.rbegin()` dereferences to the final
item of `vec`, the iterator it wraps (`vec.rbegin().base()`) is equal to
`vec.end()`.
In the remove case here, we advance `it` (backwards), erasing the item
we just advanced past by grabbing its (pre-increment) base
forward-iterator and subtracting 1.
The iterator maths here is obviously all OK, but the forward-iterator
that `it` wraps post-increment actually points to the item we just
removed. That iterator was invalidated by the `erase()` call.
That this works anyway is (AFAICT) some combination of luck and/or
promises that aren't part of the C++ spec, but MSVC's debug iterator
support picks this up.
`erase()` returns the new iterator that follows the item just erased,
which happens to be the exact one we want our reverse-iterator to wrap
for the next loop; we get a fresh iterator to the same base, now without
the preceding item.
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