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+# `<nixpkgs>/modules`
+
+This directory hosts subdirectories representing each module [class](https://nixos.org/manual/nixpkgs/stable/#module-system-lib-evalModules-param-class) for which the `nixpkgs` repository has user-importable modules.
+
+Exceptions:
+- `_class = "nixos";` modules go in the `<nixpkgs>/nixos/modules` tree
+- modules whose only purpose is to test code in this repository
+
+The emphasis is on _importable_ modules, i.e. ones that aren't inherent to and built into the Module System application.