def lookup_descriptor(self, definition_name):
    try:
      return self.__descriptors[definition_name]
    except KeyError:
      pass

    if self.__descriptor_loader:
      definition = self.__descriptor_loader(definition_name)
      self.__descriptors[definition_name] = definition
      return definition
    else:  
      raise messages.DefinitionNotFoundError(
        'Could not find definition for %s' % definition_name)
Revert a sequence creation
dataset/ETHPy150Open AppScale/appscale/AppServer/lib/protorpc/protorpc/descriptor.py/DescriptorLibrary.lookup_descriptor--crossover with--dataset/ETHPy150Open perseas/Pyrseas/tests/dbobject/test_sequence.py/SequenceUndoSqlTestCase.test_undo_create_sequence