docs: 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)