example refactor snowflake module system

nix/snow/flake/modules/overlays.nix

@@ -0,0 +1,32 @@
+{ lib, ... }:
+let
+  inherit (lib)
+    mkOption
+    types
+    ;
+in
+{
+  options = {
+    flake.overlays = mkOption {
+      # uniq -> ordered: https://github.com/NixOS/nixpkgs/issues/147052
+      # also update description when done
+      type = types.lazyAttrsOf (types.uniq (types.functionTo (types.functionTo (types.lazyAttrsOf types.unspecified))));
+      # This eta expansion exists for the sole purpose of making nix flake check happy.
+      apply = lib.mapAttrs (_k: f: final: prev: f final prev);
+      default = { };
+      example = lib.literalExpression ''
+        {
+          default = final: prev: {};
+        }
+      '';
+      description = ''
+        An attribute set of [overlays](https://nixos.org/manual/nixpkgs/stable/#chap-overlays).
+
+        Note that the overlays themselves are not mergeable. While overlays
+        can be composed, the order of composition is significant, but the
+        module system does not guarantee sufficiently deterministic
+        definition ordering, across versions and when changing `imports`.
+      '';
+    };
+  };
+}