diff --git a/src/dotfiles/registry/collection.nix b/src/dotfiles/registry/collection.nix
index b5de52d..83d08b7 100644
--- a/src/dotfiles/registry/collection.nix
+++ b/src/dotfiles/registry/collection.nix
@@ -5,21 +5,6 @@
   validate,
 }:
 let
-  withOperations =
-    registry:
-    registry
-    // {
-      join = host: entry: join registry host entry;
-    };
-
-  join =
-    registry: host: entry:
-    lib.filterAttrs (_name: value: value != null) (
-      lib.mapAttrs (
-        registryName: _registry: (registry.${registryName}.${host} or { }).${entry} or null
-      ) definitions
-    );
-
   collectData =
     let
       data = lib.mapAttrs (
@@ -74,5 +59,16 @@ let
 
   rawDeclarations =
     registryName: lib.mapAttrs (_hostName: host: host.config.registry.${registryName} or { }) hosts;
+
+  join =
+    registry: host: entry:
+    lib.filterAttrs (_name: value: value != null) (
+      lib.mapAttrs (
+        registryName: _registry: (registry.${registryName}.${host} or { }).${entry} or null
+      ) definitions
+    );
 in
-withOperations collectData
+collectData
+// {
+  join = host: entry: join collectData host entry;
+}
diff --git a/src/skills/style/nix/rules/let.md b/src/skills/style/nix/rules/let.md
index 7a3d576..222e06a 100644
--- a/src/skills/style/nix/rules/let.md
+++ b/src/skills/style/nix/rules/let.md
@@ -15,3 +15,16 @@ Do not apply this rule when a binding names a local constant used to keep siblin
 
 Shared convenience bindings hide dependency direction.
 Explicit references show which value owns a concept and which value consumes it.
+
+## Do not use single-use merge wrappers
+
+### Rule
+
+Do not bind a local function that extends an attribute-set argument when it has one caller.
+Merge the fields in the caller.
+Keep the function when two or more callers use it.
+
+### Rationale
+
+Single-use wrappers hide construction of the result.
+Direct merges show the added fields and their owner.