docs: def add_property(self,pid, label,term_span):
          """
          Adds a new property to the property layer
          @type pid: string
          @param pid: property identifier
          @type label: string
          @param label: the label of the property
          @type term_span: list
          @param term_span: list of term identifiers
          """
          new_property = Cproperty(type=self.type)
          self.node.append(new_property.get_node())
          ##Set the id
          if pid is None:
              ##Generate a new pid
              existing_pids = [property.get_id() for property in self]
              n = 0
              new_pid = ''
              while True:
                  new_pid = 'p'+str(n)
                  if new_pid not in existing_pids: break
                  n += 1
              pid = new_pid
          new_property.set_id(pid)
          
          new_property.set_type(label)
          
          new_ref = Creferences()
          new_ref.add_span(term_span)
          new_property.set_reference(new_ref)