import (fetchTarball {
   # Nixpkgs checkout from 2019-01-30.

   url = https://github.com/NixOS/nixpkgs/archive/2e2ab461b76ec2c586ab126acc53f53bce49cb94.tar.gz;
   sha256 = "17gicjn4cafpajczv49czy5r61w16kplkbz6r8hqn5rkvp4i5i9b";
})