def __len__(self):
        raise NotImplementedError('Method must be implemented by subclass')
Set the Message representation of visible headers.
dataset/ETHPy150Open babble/babble/include/jython/Lib/mailbox.py/Mailbox.__len__--crossover with--dataset/ETHPy150Open babble/babble/include/jython/Lib/mailbox.py/BabylMessage.set_visible