Shimon b. Azzai says: (If it is already written) "heed," why is it (also) written "you shall heed"? I might think that if one wishes to heed, he is caused to heed later; to forget, he is caused to forget later; it is, therefore, written "heed, you shall heed," "forget, you shall forget." How so? If he wishes to heed, he is caused to heed at once; (if he wishes) to forget, he is caused to forget at once.