Here's a rough attempt that could probably be cleaned up into something good:

@subjects=`echo \
  '<stylesheet xmlns="http://www.w3.org/1999/XSL/Transform" '		\
  'xmlns:str="http://exslt.org/strings" version="1.0" '			\
  'extension-element-prefixes="str"><output method="text"/>'		\
  '<template match="/"><for-each select="//subject/@category">'		\
  '<for-each select="str:tokenize(.)"><value-of select="."/>'		\
  '<if test="position() != last()"><text>_</text></if></for-each>'	\
  '<text> </text></for-each></template></stylesheet>' 			\
  | xsltproc - $(_DOC_OMF_IN)`;						\
for subject in $$subjects; do						\
  code=`echo $$subject | sed -e 's/|//g' -e 's/_/ /g'`;			\
  valid=`echo								\
    '<stylesheet xmlns="http://www.w3.org/1999/XSL/Transform" '		\
    'version="1.0"><output method="text"/><template match="/">'		\
    '<if test="//sect[@categorycode='"'$$code'"']">1</if>'		\
    '</template></stylesheet>' | xsltproc - 				\
  /usr/share/scrollkeeper/Templates/C/scrollkeeper_cl.xml`;		\
  if [ "x$$valid" != "x1" ]; then					\
    echo -n "Invalid OMF category code: ";				\
    echo $$subject | sed -e 's/_/ /g';					\
    exit 1;								\
  fi;									\
done;

This would be put in the rule for building $(_DOC_OMF_DB).  There are a few
issues with this:

1) I hard-coded the path to scrollkeeper_cl.xml.
2) I don't think the echo/exit business is the proper way to handle the error.
3) This will wrong for each OMF file we build, as in once for each language.  I
suppose we could just let that happen, and maybe even make each language OMF
check against the appropriate language's scrollkeeper_cl.xml file.  But if
there's actually a difference in codes between languages, we've got more serious
problems.

Since we're just verifying an input file (the .omf.in file), we don't really
need to check it for every output file it makes.  I suppose an easy solution is
just to wrap the whole check in a conditional that checks if we're outputting
the C OMF file.

(Yeah, the inline XSLT makes me cringe too.  But I'm not allowed to use XML
Starlet here.)

Under point (3), s/This will wrong/This will run/

On point (1):

`scrollkeeper-config --pkgdatadir`/Templates/C/scrollkeeper_cl.xml

That'll give us the file less stupidly.

I did this in db2omf.xsl instead.