[xsl] Getting ordering info from a lookup table

["Trevor Nicholls" <trevor@xxxxxxxxxxxxxxxxxx>]

Hello

My transform is using an external XML file as a lookup table. Among other
things, the external file contains a list:
  ...
  <files>
    <doc>file_one</doc>
    <doc>file_two></doc>
    <doc>file_three</doc>
    ...
  </files>

Each time the transform runs, the input XML file corresponds to one of the
<doc> elements in the lookup file. The input file name is passed in as a
parameter to the transformation.

  <xsl:param name="myself" />

I think the following construction can do the lookup to the external file
and locate the current doc:

  .. select="document('lookup.xml')/files/doc[. = $myself]"

but of course this is pointless by itself as I already have the doc name.
What I actually want is to obtain the names of the previous and next docs -
if they exist. There's surely an elegant way to do this, but I don't know
it. What can I do?

Thanks
Trevor