xsl grouping problem

I've been chewing on this for some time now and would love some suggestions.
Coming from a document-centric environment, I am given xml that looks like this:

<chapter>
<para>This is a paragraph in the chapter</para>
<line_first>This is first line of a stanza of poetry</line_first>
<line>This is line of poetry</line>
<line>This is line of poetry</line>
<line>This is line of poetry</line>
<line_last>This is last line of a stanza of poetry</line_last>
<para>This is a paragraph in the chapter</para>
</chapter>

The challenge is to perform copy and grouping operations to a) copy all existing
structure as is, using copy-of, which is not a problem. However, to the above
xml I wish to apply additional structure to create a poem stanza:

<chapter>
<para>This is a paragraph in the chapter</para>
<stanza>
<line_first>This is first line of poetry</line_first>
<line>This is line of poetry</line>
<line>This is line of poetry</line>
<line>This is line of poetry</line>
<line_last>This is last line of poetry</line_last>
</stanza>
<para>This is a paragraph in the chapter</para>
</chapter>

My difficulty stems from being dealt two dissimilar element names at the
beginning and end of the block of xml that needs to be grouped. My attempts to
do this with group-starting-with and group-ending-with have failed. Performing
group-adjacent on the <line> elements miss the first_ and last_line elements.
Any thoughts?

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