Parse and add a group requirement for this trigger
        :param element: The XML Element object
        :type  element: etree._Element

def _parse_group(self, element):
        """
        Parse and add a group requirement for this trigger
        :param element: The XML Element object
        :type  element: etree._Element
        """
        self._log.debug('Adding Trigger group: {group}'.format(group=element.text))
        if isinstance(self.groups, set):
            self.groups.add(element.text)
        elif self.groups is None:
            self.groups = {element.text}
        else:
            raise TypeError('Unrecognized group type, {type}: {groups}'
                            .format(type=type(self.groups), groups=str(self.groups)))