def node_before_callbacks!
      validate_unpersisted_nodes!
      from_node.conditional_callback(:create, from_node.persisted?) do
        to_node.conditional_callback(:create, to_node.persisted?) do
          yield
        end
      end
    end
def node_before_callbacks!
      validate_unpersisted_nodes!
      from_node.conditional_callback(:create, from_node.persisted?) do
        to_node.conditional_callback(:create, to_node.persisted?) do
          yield
        end
      end
    end
Node callbacks only need to be executed if the node is not persisted. We let the `conditional_callback` method do the work,
   we only have to give it the type of callback we expect to be run and the condition which, if true, will prevent it from executing.