Example content:

<prototype name="key" return-type="node-set">
<arg type="string"/>
<arg type="object"/>
</prototype>

<function>key</function>

Example Use

<xsl:key name='func' match='prototype' use='@name'/>

<xsl:template match="function">
<p>The return type is 
<xsl:value-of select="key('func',.)/@return-type"/>
</p>
</xsl:template>

Which would give you:

<p>The return type is node-set</p>