docs: def get_id_head(self):
          '''
          Returns the id  of the target that is set as "head"
          @rtype: string
          @return: the target id (or None) of the head target
          '''
          id_head = None
          for target_node in self:
              if target_node.is_head():
                  id_head = target_node.get_id()
                  break
          return id_head