docs: def add_edge(self, edge):
          """
          Adds Edges to the probnet dict.
  
          Parameters
          ----------
          edge: <Element Link at Links Node in XML>
              etree Element consisting Variable tag.
  
          Examples
          -------
          >>> reader = ProbModelXMLReader()
          >>> reader.add_edge(edge)
          """
          var1 = edge.findall('Variable')[0].attrib['name']
          var2 = edge.findall('Variable')[1].attrib['name']
          self.probnet['edges'][(var1, var2)] = {}
          self.probnet['edges'][(var1, var2)]['directed'] = edge.attrib['directed']
          # TODO: check for the case of undirected graphs if we need to add to both elements of the dic for a single edge.
          if edge.find('Comment') is not None:
              self.probnet['edges'][(var1, var2)]['Comment'] = edge.find('Comment').text
          if edge.find('Label') is not None:
              self.probnet['edges'][(var1, var2)]['Label'] = edge.find('Label').text
          if edge.find('AdditionalProperties/Property') is not None:
              self.probnet['edges'][(var1, var2)]['AdditionalProperties'] = {}
              for prop in edge.findall('AdditionalProperties/Property'):
                  self.probnet['edges'][(var1, var2)]['AdditionalProperties'][prop.attrib['name']] = prop.attrib['value']