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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 observable_object_keys(instance):
 """Ensure observable-objects keys are non-negative integers. """

    digits_re = re.compile(r"^\d+$")
    for key in instance['objects']:
        if not digits_re.match(key):
            yield JSONError("'%s' is not a good key value. Observable Objects "
                            "should use non-negative integers for their keys."
                            % key, instance['id'], 'observable-object-keys')