This is a little tricky to explain. So, ``limited`` can return a concrete limited collection class like ``<stretchy-byte-vector>`` directly, or it can return a limited collection class specifier like ``<limited-stretchy-vector-type>``. ``<stretchy-byte-vector>`` is defined with certain assumptions: it has an element type of ``<byte>`` and it has a default fill value of ``0``. If the user instantiates the type directly, that's what he'll get.

If he wants a different default fill value, though, he'll pass something other than ``0`` to ``limited``. This causes the ``fully-specified?`` variable here to be ``#f``, and ``limited`` will return the limited collection class specifier, ``<limited-stretchy-vector-type>``, instead of the actual concrete collection class, ``<stretchy-byte-vector>``.

Then when the user later instantiates the limited collection class specifier, he will still get an instance of ``<stretchy-byte-vector>``, but it will have the default fill value set to what he wanted instead of ``0``. The limited collection class specifier carried that value over from the ``limited`` call.