itstreams need not all occur contiguously (although a
specific media mapping may require this; it is not mandated by the
generic Ogg stream spec). Terminal pages may be 'nil' pages,
that is, pages containing no content but simply a page header with
position information and the 'last page of bitstream' flag set in the
page header.</p>

<p>Each grouped bitstream must have a unique serial number within the
scope of the physical bitstream.</p>

<h3>sequential and concurrent multiplexing</h3>

<p>Groups of concu