group(dag_node):
   if dag_node.num_children == 0:
      new_node = new DagNode(dag_node.value)
   if dag_node.num_children > 1:
      new_node = new DagNode(dag_node.value)
      new_node.children = [group(child) for child in dag_node.children]
   if tree_node.num_children == 1:
      value_pair = (dag_node.value, dag_node.children[0].value)
      new_node = new DagNode(value_pair)
      new_node.children = dag_node.children[0].children
   return new_node