docs: def toElkJson_registerPorts(self, idStore):
          """
          The index of a port in the fixed order around a node.
          The order is assumed as clockwise, starting with the leftmost port on the top side.
          This option must be set if ‘Port Constraints’ is set to FIXED_ORDER
          and no specific positions are given for the ports. Additionally,
          the option ‘Port Side’ must be defined in this case.
          """
          addIndex = self.portConstraints == PortConstraints.FIXED_ORDER
          for i, p in enumerate(self.iterPorts()):
              if addIndex:
                  p.index = i
              idStore.registerPort(p)
  
          for ch in self.children:
              ch.toElkJson_registerPorts(idStore)