[xsl] Merging structure and content by own id with XSLT 2.0

[Jacobus Reyneke <jacobusreyneke@xxxxxxxxx>]

Good day,

I need to merge new data into an existing data set. Data is identified
by data_id's.
How does one merge the structure, content and attributes of XML files?

This is a complex question, and I don't expect anyone to answer it in
full. I'm quite willing to spend a couple of days on it. I am
basically asking for some advise to get me down the right track.
Obviously I'm not going to cry if an XSL guru can type out bits of the
required XSL or just jot down some pseudo code. My greatest concern
with XSL is getting the flow right. Coming from a C background this
functional approach is still getting the better of me.

Original data:
<a data_id="1">
  AAA
</a>
<b data_id="2">
  BBB
  <c data_id="3">
    CCC
  </c>
  <e data_id="5">
    EEE
  </e>
</b>
<d data_id="4" width="2" color="blue">
DDD
</d>

New data:
<d-d data_id="4" color="green">
  D-D-D
  <c data_id="3">
    CCC
  </c>
</d-d>

XSLT
???

Expected result:
<a data_id="1">
  AAA
</a>
<b data_id="2">
  BBB
  <e custom_id="5">
    data
  </e>
</b>
<dd data_id="4" width="2" color="green">
  D-D-D
  <c data_id="3">
    CCC
  </c>
</dd>

The rules are:
- data_id's are unique in the result set
- new data content replaces original content
- new data structure modifies original structure
- new attributes updates or adds to original element attributes
- element names can be updated by new element names because only
data_id's are used
- all original elements with data_id's that are not in the new data,
remains where they are, as they are

Kind regards,
Jacobus