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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 titles(self, key, value):
 """Populate the ``titles`` key."""

    if not key.startswith('245'):
        return {
            'source': value.get('9'),
            'subtitle': value.get('b'),
            'title': value.get('a'),
        }

    self.setdefault('titles', []).insert(0, {
        'source': value.get('9'),
        'subtitle': value.get('b'),
        'title': value.get('a'),
    })