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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def vocab_marking_definition(instance):
 """Ensure that the `definition_type` property of `marking-definition` objects is one of the values in the STIX 2.0 specification. """

    if (instance['type'] == 'marking-definition' and
            'definition_type' in instance and not
            instance['definition_type'] in enums.MARKING_DEFINITION_TYPES):

        return JSONError("Marking definition `definition_type` should be one "
                         "of: %s." % ', '.join(enums.MARKING_DEFINITION_TYPES),
                         instance['id'], 'marking-definition-type')