Syntax of PIs

(please excuse an ignorant question)

I am confused about the syntax of PIs (or at least markup which 
begins with the '<?' string.  Sometimes it is closed with a '?>'
and sometimes only with a '>'

For example here is some recent correspondence from several WG members

> 
>    <?XHL link=a,footnote>
> 
>    <?XML LINK ELEMENTS "a footnote">
> 
[...]
> 
>    <?XML ATTLIST PrimaryIE
>         xml             (linkto)        "linkto"
>         xmlnames        CDATA           #FIXED "linkends ptr"
>         scheme          (intid)         "intid"
>    ?>
> 
>     <?-XML- URI element A attribute HREF?>
>     <?-XML- URI element FOOTNOTE attribute HREF?>
>     <?-XML- SYSTEM element IMAGE attribute SRC action embed?>
>     <?-XML-XHL- -XML-XHL-URI- -XML-XHL-ELEMENT A ..... ugh?>
> 

In the draft (2.5) <9 Processing instructions> it seems clear that
'?>' is required but in 2.8 <12 ...> it gives
<?XML encoding="UTF-8">
whereas in 2.9 <13 ...> and 3.5 <25...> a closing '?>' is required.

Peter

-- 
Peter Murray-Rust, (domestic net connection)
Virtual School of Molecular Sciences, Nottingham University, UK
http://www.ccc.nottingham.ac.uk/~pazpmr/