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.

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)