Add a (syntax category) <node> to the document graph.

        Parameters
        ----------
        node : etree.Element
            etree representation of a <node> element
            A <node> describes an element of a syntax tree.
            The root <node> element does not have a parent attribute,
            while non-root nodes do

        Example
        -------
        <node xml:id="s1_505" cat="SIMPX" func="--">
            <node xml:id="s1_501" cat="LK" func="-" parent="s1_505">

            # this is the root of the syntax tree of the sentence, but
            # it is not the root node of the sentence, since there might
            # be nodes outside of the tree which are children of the
            # sentence root node (e.g. <word> elements representing a
            # quotation mark)

def add_node_element(self, node):
        """Add a (syntax category) <node> to the document graph.

        Parameters
        ----------
        node : etree.Element
            etree representation of a <node> element
            A <node> describes an element of a syntax tree.
            The root <node> element does not have a parent attribute,
            while non-root nodes do

        Example
        -------
        <node xml:id="s1_505" cat="SIMPX" func="--">
            <node xml:id="s1_501" cat="LK" func="-" parent="s1_505">

            # this is the root of the syntax tree of the sentence, but
            # it is not the root node of the sentence, since there might
            # be nodes outside of the tree which are children of the
            # sentence root node (e.g. <word> elements representing a
            # quotation mark)

        """
        node_id = self.get_element_id(node)
        if 'parent' in node.attrib:
            parent_id = self.get_parent_id(node)
        else:
            # <node> is the root of the syntax tree of a sentence,
            # but it might be embedded in a <edu> or <edu-range>.
            # we want to attach it directly to the <sentence> element
            parent_id = self.get_sentence_id(node)
        self.add_node(node_id, layers={self.ns, self.ns+':syntax'},
                      attr_dict=self.element_attribs_to_dict(node),
                      label=node.attrib['cat'])
        self.add_edge(parent_id, node_id, edge_type=dg.EdgeTypes.dominance_relation)