[xsl] Including an attribute discriminator in the for-each-group's grou

I have two elements that differ only by attribute. My for-each-group code 
currently considers them to be identical, so I need to distinguish them by 
their attributes as well as their text node. I do not know enough about 
XPath yet to construct a proper current-grouping-key(). (I'm reading Michael 
Kay's 4th edition, but have not covered the XPath section yet). Everything I 
have tried with | @* has failed.
The two elements that need to be distinguished are:

<Heading entry="author">Hahn, Henry</Heading>
<Heading entry="subject">Hahn, Henry</Heading>

The code snippet is:

     <xsl:for-each-group select="Item" group-by="Heading" 
collation="{$sorting-collation}">
       <xsl:sort select="current-grouping-key()" 
collation="{$sorting-collation}"/>

Thanks,
Mark 

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