[xsl] Find several different attribute values on many elements

["dvint@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

I'm trying to process a collection of documents and extract the 
distinct-values for several attributes. My first attempt was this which 
is very slow

	<xsl:variable name="ditacollectionString"
		select="concat($srcPath, '?select=*.dita')"/>

	<xsl:variable name="xmlcollectionString"
		select="concat($srcPath, '?select=*.xml')"/>

		<xsl:for-each
			select="distinct-values(collection($ditacollectionString)//*/@audience)" 
>
			<xsl:value-of select="concat(., ' ')"/>
		</xsl:for-each>
		<xsl:for-each
			select="distinct-values(collection($ditacollectionString)//*/@platform)" 
>
			<xsl:value-of select="concat(., ' ')"/>
		</xsl:for-each>
		<xsl:for-each
			select="distinct-values(collection($ditacollectionString)//*/@props)" 
>
			<xsl:value-of select="concat(., ' ')"/>
		</xsl:for-each>

I'm looking for a way to just make a single for-each loop and tried this

			<xsl:for-each
				select="distinct-values(collection($ditacollectionString)//element())" 
>
				<xsl:apply-templates mode="conditions"/>

			</xsl:for-each>

	<xsl:template match="@product | @audience " mode="conditions">
		<xsl:value-of select="concat(., ' ')"/>
	</xsl:template>

But I get an error on the <xsl:apply-templates mode="conditions"/> 
indicating
The required item type of the context item is node(), but the supplied 
expression {.} has item type xs:anyAtomicType.

I also need to do this with both .dita and .xml files and I'm planning 
on making 2 collections, is there a way to do one collection()?