gistfile1.txt

{
  nixpkgs-18.09 = builtins.fetchGit {
    url = "https://github.com/nixos/nixpkgs-channels";
    ref = "nixos-18.09";
    rev = "....";
  };
  nixpkgs-19.03 = builtins.fetchGit {
    url = "https://github.com/nixos/nixpkgs-channels";
    ref = "nixos-19.03";
    rev = "....";
  };
  nixpkgs-unstable = builtins.fetchGit {
    url = "https://github.com/nixos/nixpkgs-channels";
    ref = "nixpkgs-unstable";
    rev = "....";
  };
  nixpkgs-stable = nixpkgs-18.09
  nixpkgs = nixpkgs-stable
}