The point of a reference type is to behave just like the referred-to
type. So, a Foo& should behave just like a Foo.
In these cases, we had a const Vector. If it was a const Vector of Foo,
iterating over the Vector would only permit taking const references to
the individual Foos.
However, we had a const Vector of Foo&. The behavior should not
change. We should still only be permitted to take const references to
the individual Foos. Otherwise, we would be allowed to mutate the
individual Foos, which would mutate the elements of the const Vector.
This wouldn't modify the stored pointers, but it would modify the
objects that the references refer to. Since references should be
transparent, this should not be legal.
So it should be impossible to get mutable references into a const
Vector. Since we need mutable references in these cases to call the
mutating member functions, we need to mark the Vector as mutable as
well.
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