urn:publicid: as a publicid should be justified

I think we need to insert a sentence into 7.1.1, 4th graf, as
follows:

If the public identifier is a URN in the publicid namespace
([urn:publicid]), it is converted into <del>a</del><ins>another</ins>
public identifier by "unwrapping" the URN (Section 6.4).  <ins>This
is done so that an URN can be specified as the public identifier
and an URL as the system identifier, in the absence of widely
deployed URN-resolution facilities.</ins>

-- 
There is / one art             || John Cowan <>
no more / no less              || http://www.reutershealth.com
to do / all things             || http://www.ccil.org/~cowan
with art- / lessness           \\ -- Piet Hein