[...] my "true" identity template for XSLT 2.0,
http://www.xmlplease.com/identity-template.xsl,
explained in this tutorial:
http://www.xmlplease.com/identity-template.

The "true identity template" (an xslt stylesheet of
several templates) is 500 lines long!

    cp orig.xml copy.xml        # UNIX
    copy orig.xml copy.xml      # Windows

Of course, you'd add your modifications, which you
can't do with filesystem utilities.

But this verbatim identity approach bypasses the XDM
(XPath Data Model), which XSLT operates on. It loads
the input as unparsed text and implements a new parser
using string operations.