Mulling more...
Let's say an aggregator is putting these two entries into the same
aggregate feed:
<feed ... xmlns:a="foo" xmlns:b="bar">
...
<entry>
[signature]
<a:foo ... />
<b:bar ... />
...
</entry>
</feed>
<feed ... xmlns:b="foo" xmlns:a="bar">
...
<entry>
[signature]
<b:foo ... />
<a:bar ... />
...
</entry>
</feed>
Perhaps a reasonable way to deal with the namespace prefix conflict
would be for the signature to be applied after a transform that yielded
this (putting full namespace names in where the prefixes were):
<[atom's namespace]:entry>
[signature]
<foo:foo ... />
<bar:bar ... />
...
</[atom's namespace]:entry>
Unprefixed attributes would naturally remain unprefixed, but elements
in the default namespace would need to have their namespace names
prepended.