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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 unschedule(ident):
    '''Unschedule an harvesting on a source'''
    source = get_source(ident)
    if not source.periodic_task:
        msg = 'Harvesting on source {0} is ot scheduled'.format(source.name)
        raise ValueError(msg)

    source.periodic_task.delete()
    signals.harvest_source_unscheduled.send(source)
    return source