[lxml] ElementTree as secondary input to transformation

Hi all, I wish for two separate ElementTree objects to serve as input for the same XSLT transformation. As I understand, XSLT in general supports secondary input documents by way of the XPath doc() function. Normally the function represents an XML tree from a named file. It appears that lxml supports custom resolvers in case the document is not simply a file on disk as named. Further, it appears that a custom resolver may resolve a URI as string, another filename, or a file object. What about the case of the secondary document being an ElementTree in memory? Is this case supported? Thanks. Sincerely, Eric Levy