lecture 16 - https://colab.research.google.com/drive/1XZQPXJnVHubCRiYntOY4bXXdnYx6qMGc?usp=sharing
lecture 17 - https://colab.research.google.com/drive/1zQYHCgxymGRppURWs7NPjen96suKjNU4?usp=sharing
lecture 18 - https://colab.research.google.com/drive/105rQagFROl7v7IjQsuheTstUDPDeME2D?usp=sharing