[xsl] help with an 'or'

[Jennifer Pollard <jp337@xxxxxxxxx>]

Hi, all :)

I have done a bit of searching on this question but I'm new to XSL, so 
I'm not sure what it is that I'm looking *for* exactly.

One of our projects is using XSL to create options in a select field:

-----------------------------------------------

<xsl:element name="option">

  <xsl:attribute name="value"><xsl:value-of
	select="$loop-variable"/></xsl:attribute>
  <xsl:if test="$loop-variable = $curpage">
       <xsl:attribute name="selected">true</xsl:attribute>
  </xsl:if>

<xsl:choose>
<xsl:when test="$loop-variable = 1">
   <xsl:value-of select="translate(translate(substring-after
		(//facsimile/surface[@n=$loop-variable]/@xml:id,
		'f_'),'_',' '),'_',' ')"/>
</xsl:when>

<xsl:when test="($loop-variable = $loop-limit) and
	((($loop-variable * 2) - 2) mod 2 = 0)">
   <xsl:value-of select="translate(substring-after
	(//facsimile/surface[@n=(($loop-variable * 2) - 2)]/@xml:id,
	'f_'),'_',' ')"/>
</xsl:when>

<xsl:otherwise>
   <xsl:value-of select="translate(substring-after
	(//facsimile/surface[@n=(($loop-variable * 2) - 2)]/@xml:id,
	'f_'),'_',' ')"/> -
   <xsl:value-of select="translate(substring-after
	(//facsimile/surface[@n=(($loop-variable * 2) - 1)]/@xml:id,
	'f_'),'_',' ')"/>
</xsl:otherwise>
</xsl:choose>

</xsl:element>

-----------------------------------------------

The problem is that in the 'value-of select' statements, it's limiting 
the list to display only values that start with f_, which is fair enough 
- I can see where it's meant to be doing that.  What I need it to do, 
though, is give me anything that starts with either f_ OR p_.

I'm not sure how to insert any sort of 'or' clause into those statements 
and still retain their ability to do what we need them to do (sometimes 
we get both f_ and _p on the same line.  For an example of a foliated 
book, see: 
<http://scriptorium.english.cam.ac.uk/manuscripts/images/openings.php?ms=S.54&page=2>

The bit I'm trying to modify is the drop down next to the word 
'Opening'.  Most of the books in the collection are foliated (hence 
pages starting with f_) but a recent acquisition is paginated.  What 
happens is that we get blanks instead of page numbers when the xmlid 
consists of p_<some number>.  I have modified the code above so that 
page 1 and 2 are f_ and the rest are p_, but because this code is reused 
with all books, that does break the foliated ones. (Which I expected: I 
was just checking to see it worked at all! ;)

Can anyone lend me a hand with this?  Apologies if I've been unclear - 
I'll do my best to answer any questions you might have.

-Jen