def validate(self):
        """ Checks whether the backbone string is correct.
        """
        allowed_backbones = ['vgg16', 'vgg19']

        if self.backbone not in allowed_backbones:
            raise ValueError('Backbone (\'{}\') not in allowed backbones ({}).'.format(self.backbone, allowed_backbones))
 Checks whether the backbone string is correct.
        
keras_retinanet/models/vgg.py