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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 license2marc(self, key, value):
 """Populate the ``540`` MARC field."""

    return {
        'a': value.get('license'),
        'b': value.get('imposing'),
        'u': value.get('url'),
        '3': value.get('material'),
    }