PR on unit order

Hi David, all,

I noticed this change in the latest draft (sub-flows section):

Old: "User agents coming later in the process MUST keep the <unit> elements in their initial order."

Is now:

New: "Writers MUST preserve the order of <unit> elements as initially set by the Extractor."

I think the old version was better:

a) it's a general PR rather than specific to writer: 

b) you can only ask an agent to keep the order it found in the document it read: it cannot know if this is the extractor order or
not. (and if all agents respect this PR we will keep the extractor initial order)
So maybe something like: "A agent MUST keep the <unit> elements in the same order it found them initially."

That PR should probably be elsewhere than the sub-flows section: it's a general PR.

Cheers,
-ys