anchor.diff:  
         # cudaSupport causes fetch of ncclArchive, resulting in different hashes
         sha256 = if cudaSupport then
  -        "09j57w6kc0vkfcdwr0qggy3qgrgq82kfa2jrwvvcnij4bl3wj40j"
  +        "1bi6aydidgi943hiqj0d279jbz2g173hvafdqla1ifw2qdsm73pb"
         else
  -        "14g8z49qz7d8n1c2mcsfhr7yqpcy7mhmpm6hgmqvpgb8vm7yvwrc";
  +        "0l5510fr8n22c4hx9llr0vqqhx9wlgkyxl55fxbixhssd0ai05r4";
       };
   
       buildAttrs = {
  
positive.code: 
    tfFeature = x: if x then "1" else "0";
  
    version = "1.15.2";
    variant = if cudaSupport then "-gpu" else "";
    pname = "tensorflow${variant}";
  
  
negative.code:       # include security vulnerabilities. So we make it optional.
        # https://github.com/tensorflow/tensorflow/issues/20280#issuecomment-400230560
        sed -i '/tensorboard >=/d' tensorflow/tools/pip_package/setup.py
  
        substituteInPlace tensorflow/tools/pip_package/setup.py \
          --replace "numpy >= 1.16.0, < 1.19.0" "numpy >= 1.16.0"
      '';
  
      preConfigure = let
  