Add a child node to the current node instance.

        :param node: the child node instance.
        :type node: Node
        :returns: The new child node instance.   
        :rtype: Node

def add_child(self, node):
        """Add a child node to the current node instance.

        :param node: the child node instance.
        :type node: Node
        :returns: The new child node instance.   
        :rtype: Node 
        """
        if not issubclass(node.__class__, Node):
            raise TypeError("{}.add_child: arg «node»=«{}», type {} not valid.".format(self.__class__.__name__, node, type(node)))
        self.childs.append(node)
        node.parent = self
        return node