docs: def push_group(self, my_dict, key, element):
          ''' Push a group as a child of another group. '''
          parent_group = my_dict.setdefault(key, {})
          if not isinstance(parent_group, dict):
              parent_group = my_dict[key] = {'hosts': parent_group}
          child_groups = parent_group.setdefault('children', [])
          if element not in child_groups:
              child_groups.append(element)