Returns the proxy from the last node visited on the path, or `None`,
        if no node has been visited yet.

def parent(self):
        """
        Returns the proxy from the last node visited on the path, or `None`,
        if no node has been visited yet.
        """
        if len(self.nodes) > 0:
            parent = self.nodes[-1].proxy
        else:
            parent = None
        return parent