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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 register_classes(yaml: ruamel.yaml.YAML, classes: Optional[Iterable[Any]] = None) -> ruamel.yaml.YAML: """ Register externally defined classes. """

    # Validation
    if classes is None:
        classes = []

    # Register the classes
    for cls in classes:
        logger.debug(f"Registering class {cls} with YAML")
        yaml.register_class(cls)

    return yaml