Parses <OnEvent>

        @param node: Node containing the <OnEvent> element
        @type node: xml.etree.Element

def parse_on_event(self, node):
        """
        Parses <OnEvent>

        @param node: Node containing the <OnEvent> element
        @type node: xml.etree.Element
        """

        try:
            port = node.lattrib['port']
        except:
            self.raise_error('<OnEvent> must specify a port.')
            
        event_handler = OnEvent(port)

        self.current_regime.add_event_handler(event_handler)

        self.current_event_handler = event_handler
        self.process_nested_tags(node)
        self.current_event_handler = None