new_contents: from zope.interface.verify import verifyObject, verifyClass
  from zope.interface.exceptions import DoesNotImplement
  
  from IOperation import IOperation
  from utils import Plus
  
  
  assert verifyClass(IOperation, Plus) == True
  assert verifyObject(IOperation, Plus()) == True
  
  try:
      verifyObject(IOperation, object())
  except DoesNotImplement:
      pass
  else:
      raise RuntimeError("Should raise DoesNotImplement because there is no implementation")
  
subject: Create verifier.py