substitute variable value into xpath expression - prob

This is using MSXSL4.

near the top of the stylesheet
*------------------------------------
<xsl:variable name="XML_PARENT" select="'training_types'" />
<xsl:variable name="XML_CHILD" select="'training_type'" />

down in the body
*--------------------
<xsl:for-each
select="msxsl:node-set($TRAINING_TYPE_XMLDOC)/$XML_PARENT/$XML_CHILD">

The TRAINING_TYPE_XMLDOC is fine - it's the XML_PARENT and XSL_CHILD that I'm
having trouble with.  All I'm really trying to do is substitute in the literal
values of the XSL_PARENT and XSL_CHILD into the XPath select.  Here is the
error I'm getting:

*------------------
NodeTest expected here.
msxsl:node-set($TRAINING_TYPE_XMLDOC)/-->$<--XML_PARENT/$XML_CHILD
*------------------

I'm sure it's something stupid I'm doing/not doing - please let me know what
it is.

Thanks.

Hardy Merrill