ordering of nodes

Take this xml:
<book>
<para>
This paragraph to be in order as encountered for
both target document A and B
</para>
<para b="2">
This paragraph to be in order as encountered for target A
but to appear after the subsequent paragraph in target B
</para>
<para b="1">
Another paragraph
</para>
<para>
And however many others
</para>
...
</book>

Based on the value of a variable (which I use to define various
output types), I want the order to be either as encountered in the
xml or with (say) those two paragraphs transposed.

OF course the ordering can be a lot more complex but the above
example should give the idea.  Now, I do not want to have to give
each paragraph say a unique index number say in an attribute because
it may sometimes only be an occasional transposition or two that I
require.  In the above requirement, I only apply attributes to the
two nodes.

How do I order them?
-- 
Eric Smith - currently xalan and fop on linux

 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list