{
  inputs = { nixpkgs.url = "github:NixOS/nixpkgs/nixos-20.03"; };
  outputs = { self, nixpkgs, }:
    let

      inherit (builtins) attrNames;
      inherit (nixpkgs.lib) genAttrs;

      for-supported-systems = genAttrs (attrNames
        ((import (nixpkgs + "/pkgs/top-level/release.nix")
          { }).stdenvBootstrapTools));

    in {

      overlay = final: prev: { git-cache = import ./. { pkgs = final; }; };
      overlays = { git-cache = self.overlay; };

      packages = for-supported-systems (system: {
        inherit (nixpkgs.legacyPackages."${system}".appendOverlays
          [ self.overlay ])
          git-cache;
      });

      defaultApp = for-supported-systems (system: {
        type = "app";
        program = "${self.packages."${system}".git-cache}/bin/git-cache";
      });
    };
}