Adding or modifying by path

First, thanks to the extraordinary efforts of Jeni Tennison, David Pawson,
Michael Kay and everyone who has participated on this list.  I would not
have been able to learn XSLT as it evolves without you.

Now that the gushing is out of the way, I'm desperate for an approach to add
an element to the result tree (which is otherwise a copy of the input tree)
without adding any unneeded "duplicate" elements.  E.g.:

adding /a/b/c with a value of 'foo' to input:

<a>
 <b>
  <c>42</c>
 </b>
</a>

or to:

<a></a>

should *both* prepare:

<a>
 <b>
  <c>foo</c>
 </b>
</a>

with any other content and structure preserved.  This is trivial to
describe, so I'm hoping to feel like a fool when someone shows me how
trivial it is to implement :-)

It should not be relevant, but along the lines of full disclosure: this
sheet is to be the result of a different sheet, which takes "/a/b/c" and
"foo" as input, producing the "add this element" XSLT I seek now (among
other things).

Thanks in advance!
---
Hans Gerwitz
hans @ phobia.com


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