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diff --git a/third_party/default.nix b/third_party/default.nix
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+++ b/third_party/default.nix
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+# This file defines the root of all external dependency imports (i.e.
+# third-party code) in the TVL package tree.
+#
+# There are two categories of third-party programs:
+#
+# 1) Programs in nixpkgs, the NixOS package set. For these, you might
+#    want to look at //third_party/nixpkgs (for the package set
+#    imports) and //third_party/overlays (for modifications in these
+#    imported package sets).
+#
+# 2) Third-party software packaged in this repository. This is all
+#    other folders below //third_party, other than the ones mentioned
+#    above.
+
+{ pkgs, depot, ... }:
+
+{
+  # Expose a partially applied NixOS, expecting an attribute set with
+  # a `configuration` key. Exposing it like this makes it possible to
+  # modify some of the base configuration used by NixOS.
+  #
+  # This partially reimplements the code in
+  # <nixpkgs/nixos/default.nix> as we need to modify its internals to
+  # be able to pass `specialArgs`. We depend on this because `depot`
+  # needs to be partially evaluated in NixOS configuration before
+  # module imports are resolved.
+  nixos =
+    { configuration
+    , specialArgs ? { }
+    , system ? builtins.currentSystem
+    , ...
+    }:
+    let
+      eval = import (pkgs.path + "/nixos/lib/eval-config.nix") {
+        inherit specialArgs system;
+        modules = [
+          configuration
+          (import (depot.path.origSrc + "/ops/modules/default-imports.nix"))
+        ];
+      };
+
+      # This is for `nixos-rebuild build-vm'.
+      vmConfig = (import (pkgs.path + "/nixos/lib/eval-config.nix") {
+        inherit specialArgs system;
+        modules = [
+          configuration
+          (pkgs.path + "/nixos/modules/virtualisation/qemu-vm.nix")
+        ];
+      }).config;
+    in
+    {
+      inherit (eval) pkgs config options;
+      system = eval.config.system.build.toplevel;
+      vm = vmConfig.system.build.vm;
+    };
+}