Move all of NixOS to nixos/ in preparation of the repository merge

nixos/modules/misc/assertions.nix

@@ -0,0 +1,37 @@
+{ config, pkgs, ... }:
+
+with pkgs.lib;
+
+let
+
+  failed = map (x: x.message) (filter (x: !x.assertion) config.assertions);
+
+in
+
+{
+
+  options = {
+
+    assertions = mkOption {
+      default = [];
+      example = [ { assertion = false; message = "you can't enable this for that reason"; } ];
+      merge = pkgs.lib.mergeListOption;
+      description = ''
+        This option allows modules to express conditions that must
+        hold for the evaluation of the system configuration to
+        succeed, along with associated error messages for the user.
+      '';
+    };
+
+  };
+
+  config = {
+
+    # This option is evaluated always. Thus the assertions are checked as well. hacky!
+    environment.systemPackages =
+      if [] == failed then []
+      else throw "\nFailed assertions:\n${concatStringsSep "\n" (map (x: "- ${x}") failed)}";
+
+  };
+
+}