  from django.test import TestCase
  from django.db.migrations.graph import MigrationGraph, CircularDependencyError
  
  
  class GraphTests(TestCase):
      """
      Tests the digraph structure.
 
</s> remove from django.db.migrations.graph import MigrationGraph
 </s> add from django.db.migrations.graph import MigrationGraph, NodeNotFoundError </s> remove from django.utils.datastructures import OrderedSet
 </s> add  </s> remove         with self.assertRaisesMessage(ValueError, message):
 </s> add         with self.assertRaisesMessage(NodeNotFoundError, message):
from django.db.migrations.graph import CircularDependencyError, MigrationGraph, NodeNotFoundError
  from django.test import TestCase
from django.db.migrations.graph import CircularDependencyError, MigrationGraph, NodeNotFoundError
  
  
  class GraphTests(TestCase):
      """
      Tests the digraph structure.
keep replace keep keep keep keep keep
 Fixed #23556 -- Raised a more meaningful error message when migrations refer to an unavailable node 
tests/migrations/test_graph.py