I wrote:

> To test if a node has siblings:
>
>   <xsl:if test="preceding-sibling::tree or following-sibling::tree">
>
> Or:
>
>   <xsl:if test="../tree">

Of course the second one will always be true if the current node is a
<tree> node...

It would need to be modified to:

 <xsl:if test="../tree[generate-id(.) != generate-id(current())]">

to ensure the the current <tree> node doesn't get included in the
selection.