Parameters
        ----------
        secedge : etree.Element
            etree representation of a <secedge> element
        A <secEdge> element has a cat and a parent attribute,
        but inherits its ID from its parent element.
        It describes a secondary edge in a tree-like syntax representation.

        Example
        -------
           <node xml:id="s10_505" cat="VXINF" func="OV" parent="s10_507">
            <secEdge cat="refvc" parent="s10_504"/>
            <word xml:id="s10_6" form="worden" pos="VAPP" lemma="werden%passiv" func="HD" parent="s10_505" dephead="s10_7" deprel="AUX"/>
           </node>

def add_secedge(self, secedge):
        """
        Parameters
        ----------
        secedge : etree.Element
            etree representation of a <secedge> element
        A <secEdge> element has a cat and a parent attribute,
        but inherits its ID from its parent element.
        It describes a secondary edge in a tree-like syntax representation.

        Example
        -------
           <node xml:id="s10_505" cat="VXINF" func="OV" parent="s10_507">
            <secEdge cat="refvc" parent="s10_504"/>
            <word xml:id="s10_6" form="worden" pos="VAPP" lemma="werden%passiv" func="HD" parent="s10_505" dephead="s10_7" deprel="AUX"/>
           </node>
        """
        if self.ignore_secedges is False:
            edge_source = self.get_parent_id(secedge)
            edge_target = self.get_element_id(secedge)
            self.add_edge(edge_source, edge_target,
                          layers={self.ns, self.ns+':secedge'},
                          label='secedge:'+secedge.attrib['cat'],
                          edge_type=dg.EdgeTypes.pointing_relation)