def titles2marc(self, key, values):
    """Populate the ``246`` MARC field.

    Also populates the ``245`` MARC field through side effects.
    """
    first, rest = values[0], values[1:]

    self.setdefault('245', []).append({
        'a': first.get('title'),
        'b': first.get('subtitle'),
        '9': first.get('source'),
    })

    return [
        {
            'a': value.get('title'),
            'b': value.get('subtitle'),
            '9': value.get('source'),
        } for value in rest
    ]
Populate the ``246`` MARC field.

    Also populates the ``245`` MARC field through side effects.