error

$ nix-build nixos/system-tests.nix
error: attribute 'tahoe-lafs' missing, at /home/exarkun/Work/PrivateStorage/PrivateStorageio/nixos/modules/overlays.nix:4:7
(use '--show-trace' to show detailed location information)

overlays.nix

self: super: {
  privatestorage = super.python27.buildEnv.override
  { extraLibs =
    [ super.python27Packages.tahoe-lafs
      super.python27Packages.zkapauthorizer
    ];
    # Twisted's dropin.cache always collides between different                                                                                                                                                                                                                 
    # plugin-providing packages.                                                                                                                                                                                                                                               
    ignoreCollisions = true;
  };

  python27 = super.python27.override {
    packageOverrides = python-self: python-super: {
      twisted = python-super.callPackage ../pkgs/twisted.nix { };
    };
  };
}

zkap-overlays.nix

self: super: {
  openssl = super.openssl_1_1;

  ristretto = super.callPackage ./ristretto.nix { };                                                                                  

  python27 = super.python27.override {
    packageOverrides = python-self: python-super: {
      # new tahoe-lafs dependency                                                                                                                                                                                                                                              
      eliot = python-super.callPackage ./eliot.nix { };
      # new autobahn requires a newer cryptography                                                                                                                                                                                                                             
      cryptography = python-super.callPackage ./cryptography.nix { };
      # new cryptography requires a newer cryptography_vectors                                                                                                                                                                                                                 
      cryptography_vectors = python-super.callPackage ./cryptography_vectors.nix { };
      # new tahoe-lafs depends on a very recent autobahn for better                                                                                                                                                                                                            
      # websocket testing features.                                                                                                                                                                                                                                            
      autobahn = python-super.callPackage ./autobahn.nix { };

      # tahoe-lafs in nixpkgs is packaged as an application!  so we have to                                                                                                                                                                                                    
      # re-package it ourselves as a library.                                                                                                                                                                                                                                  
      tahoe-lafs = python-super.callPackage ./tahoe-lafs.nix { };

      # we depend on the privacypass python library, a set of bindings to the                                                                                                                                                                                                  
      # challenge-bypass-ristretto Rust library                                                                                                                                                                                                                                
      privacypass = python-super.callPackage ./privacypass.nix { };

      # And add ourselves to the collection too.                                                                                                                                                                                                                               
      zkapauthorizer = python-super.callPackage ./zkapauthorizer.nix { };
    };
  };
}