[DDI-SRG] Comparative

[Joachim Wackerow]

After thinking again about comparison issues, I was looking at
Comparative. Then I realized, that this must be improved.

But first of all I detected a real bug:
A REQUIRED element 'Difference' is in the element 'Difference', which
would result in an infinite XML tree of nested 'Difference'. That is a
very nice idea :).

See bug 123.

I think the correspondence for comparison items should be described with
more possibilities, more machine-actionable items should be used, and 
the internal consistency improved.

Details see bug 125

Achim