use case extensions

["mark boylan" <markboylan@hotmail.com>]

I'm creating some document templates for a software project. One of the 
templates that I have to create is a use case. One section of the use case 
is the "Main success scenario." The docbook "procedure" seems perfect for 
this. The next section of the document is "Extensions." This section is 
similar to a procedure, but the numbering of each step should refer back to 
the numbering in the "Main success scenario" section. When completed, a use 
case with one extension to step two might look like this:

Main Success Scenario
  1. Step one
  2. Step two
  3. Step three

Extensions
  2a. Exception occurs in step 2
    2a1. Handle the exception

What is the best way to create something like this using DocBook? Is a 
procedure appropriate for the extensions? Is there any way that I can have 
the extension procedures reference the main procedure? What is the best way 
to handle the numbering?

Thanks

- Mark

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