Returns the document element of the document containing the given node.
Only the SgmlDocument node class
exibits a DocumentElement property, so to find the document element
we first have to find the grove root, which we do by examining the
grove-root property of the current node. The only node which doesn't
have a grove-root property (so that the (node-property)
routine will correctly return #f -- ie, it exhibits the property, but
with the value #f) is the root node, but in that case,
(current-node) returns the grove root directly (this isn't clear
from the standard -- see the discussion on `Finding the root element' in
the dssslist archive at
<http://www.mulberrytech.com/dsssl/dssslist/>).
The subsequent calls to (node-property) default #f if
the property is not exhibited by the node. This catches the case where the
grove root doesn't have any document-element property, for example if
the grove is malformed because it resulted from a call to (sgml-parse)
with a non-existent file.
this node indicates the grove we want the document element of.
Type: singleton-node-list
The document element, or
#f if not found.