"interleaved" numbering

I have a hierarchic structure
block
  chapter
    section
      subsection

with standard legal numbering (examples):
 1 block
 2.3 chapter
 1.6.2 section
 3.1.1.2 subsection

within the subsections, there are
theorem elements, but their numbering
should be relative to the sections,
and not the subsections.
Example
  Assuming there are 2 theorem-elements in
  section 1, subsection 1
  and one theorem-element one theorem-element each in
  section 1, subsection 2
  section 2, subsection 1
  Then i want these theorems to be numbered
  1.1, 1.2, 1.3, 2.1

i have tried
 <xsl:number level="multiple"
             count="theorem|section"
             format="1.1"/>
also including various "from" values
but none have succeeded.

If my plan is possible, how do i have to do it?
(perhaps using variables?)


Thanks
  Jody
+-------------------------------------------+
 Jody Weissmann
 Computer Science Department
 University Zurich
 Winterthurerstr 190
 CH-8057 Zurich
 Phone: +41(0)1 625 43 17
 eMail: jody@xxxxxxxxxxxx
+-------------------------------------------+




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