xsltApplyStylesheetUser() removes DTD from doc

Hi all,

while investigating on a PHP bug[1], I found that
xsltApplyStylesheetUser() permanently removes the XML_DTD_NODE from the
given document (doc, 2nd parameter).  There is a respective comment[2]
in the implementation of xsltApplyStylesheetInternal() which points out
that this is done on purpose, but I wonder if this modification is
supposed to be *permanent*.  The documentation on
xsltApplyStylesheetUser()[3] is rather terse, and does not mention that
any modification to the given doc is supposed to happen, so I assumed
that doc is not modified in any way.

Could someone please clarify whether the current behavior is intentional?

Thanks,
Christoph

[1] <https://bugs.php.net/66937>
[2]
<https://github.com/GNOME/libxslt/blob/v1.1.34/libxslt/transform.c#L5894-L5897>
[3]
<http://xmlsoft.org/XSLT/html/libxslt-transform.html#xsltApplyStylesheetUser>