Add one child node to this node.

        Args:
            name (str): Name of the child.
            node (TreeMapNode): Node to add.

        Note:
            The name must **not** be in use.

def set_unique_child_node(self, name, node):
        """
        Add one child node to this node.

        Args:
            name (str): Name of the child.
            node (TreeMapNode): Node to add.

        Note:
            The name must **not** be in use.
        """
        try:
            temp = self._nodes[name]
            raise RuntimeError("Name '%s' is already used for child node" % name)
        except KeyError:
            pass

        self.set_child_node(name, node)