docs: def add_timex(self, timex_obj):
          """
          Adds a timex object to the layer.
          @type timex_obj: L{Ctime}
          @param timex_obj: the timex object
          """
          timex_id = timex_obj.get_id()
          #check if id is not already present
          if not timex_id in self.idx:
              
              timex_node = timex_obj.get_node()
              self.node.append(timex_node)
              self.idx[timex_id] = timex_node
          else:
              #FIXME: what we want is that the element receives a new identifier that
              #is not present in current element yet
              print('Error: trying to add new element with existing identifier')