docs: def _add_link(self, edge):
          """
          Adds an edge to the ProbModelXML.
          """
          edge_data = self.data['probnet']['edges'][edge]
          if isinstance(edge, six.string_types):
              edge = eval(edge)
          link = etree.SubElement(self.links, 'Link', attrib={'var1': edge[0], 'var2': edge[1],
                                                              'directed': edge_data['directed']})
          try:
              etree.SubElement(link, 'Comment').text = edge_data['Comment']
          except KeyError:
              pass
          try:
              etree.SubElement(link, 'Label').text = edge_data['Label']
          except KeyError:
              pass
          try:
              self._add_additional_properties(link, edge_data['AdditionalProperties'])
          except KeyError:
              etree.SubElement(link, 'AdditionalProperties')