docs: 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