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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 getElementsByClassName(start_node: ParentNode, class_name: str ) -> NodeList: """Get child nodes which has ``class_name`` class attribute."""

    classes = set(class_name.split(' '))
    return getElementsBy(
        start_node,
        lambda node: classes.issubset(set(node.classList))
    )