Parameters
        ----------
        edu : etree.Element
            etree representation of a <edu> element
            (annotates an EDU)
            Note: the arg1 EDU has a discRel child, the arg2 doesn't

        Example
        -------
        <edu xml:id="edu_55_21_1">
         <discRel relation="Explanation-Cause" marking="-|*um zu" arg2="edu_55_21_2"/>
         <word xml:id="s905_9" form="und" pos="KON" lemma="und" func="-" parent="s905_526" dephead="s905_3" deprel="KON"/>
         <node xml:id="s905_525" cat="FKONJ" func="KONJ" parent="s905_526" span="s905_10..s905_19">

        ...

       <edu xml:id="edu_55_21_2" span="s905_14..s905_20">
        <node xml:id="s905_524" cat="NF" func="-" parent="s905_525">

def add_edu(self, edu):
        """
        Parameters
        ----------
        edu : etree.Element
            etree representation of a <edu> element
            (annotates an EDU)
            Note: the arg1 EDU has a discRel child, the arg2 doesn't

        Example
        -------
        <edu xml:id="edu_55_21_1">
         <discRel relation="Explanation-Cause" marking="-|*um zu" arg2="edu_55_21_2"/>
         <word xml:id="s905_9" form="und" pos="KON" lemma="und" func="-" parent="s905_526" dephead="s905_3" deprel="KON"/>
         <node xml:id="s905_525" cat="FKONJ" func="KONJ" parent="s905_526" span="s905_10..s905_19">

        ...

       <edu xml:id="edu_55_21_2" span="s905_14..s905_20">
        <node xml:id="s905_524" cat="NF" func="-" parent="s905_525">
        """
        edu_id = self.get_element_id(edu)
        edu_attribs = self.element_attribs_to_dict(edu) # contains 'span' or nothing
        self.add_node(edu_id, layers={self.ns, self.ns+':edu'}, attr_dict=edu_attribs)

        edu_token_ids = []
        for word in edu.iterdescendants('word'):
            word_id = self.get_element_id(word)
            edu_token_ids.append(word_id)
            self.add_edge(edu_id, word_id, layers={self.ns, self.ns+':edu'},
                          edge_type=dg.EdgeTypes.spanning_relation)

        self.node[edu_id]['tokens'] = edu_token_ids