diff --git a/README.md b/README.md
index 77f9227..907a73a 100644
--- a/README.md
+++ b/README.md
@@ -40,6 +40,30 @@ eachSystem ["x86_64-linux"] (system: { hello = 42; })
 
 A small utility that builds the structure expected by the special `apps` and `defaultApp` prefixes.
 
+### `flattenTree -> attrs -> attrs`
+
+Nix flakes insists on having a flat attribute set of derivations in
+various places like the `packages` and `checks` attributes.
+
+This function traverses a tree of attributes (by respecting
+recurseIntoAttrs) and only returns their derivations, with a flattened
+key-space.
+
+Eg:
+```nix
+flattenTree { hello = pkgs.hello; gitAndTools = pkgs.gitAndTools }
+```
+Returns:
+
+```nix
+{
+  hello = «derivation»;
+  gitAndTools_git = «derivation»;
+  gitAndTools_hub = «derivation»;
+  # ...
+}
+```
+
 ## Example
 
 Here is how it looks like in practice:
diff --git a/default.nix b/default.nix
index fcc17f5..26eced0 100644
--- a/default.nix
+++ b/default.nix
@@ -47,6 +47,27 @@ let
       type = "app";
       program = "${drv}${exePath}";
     };
+
+  # Nix flakes insists on having a flat attribute set of derivations in
+  # various places like the `packages` and `checks` attributes.
+  #
+  # This function traverses a tree of attributes (by respecting
+  # recurseIntoAttrs) and only returns their derivations, with a flattened
+  # key-space.
+  #
+  # Eg:
+  #
+  #   flattenTree { hello = pkgs.hello; gitAndTools = pkgs.gitAndTools };
+  #
+  # Returns:
+  #
+  #   {
+  #      hello = «derivation»;
+  #      gitAndTools_git = «derivation»;
+  #      gitAndTools_hub = «derivation»;
+  #      # ...
+  #   }
+  flattenTree = tree: import ./flattenTree.nix tree;
 in
 {
   inherit
@@ -55,4 +76,5 @@ in
     eachSystem
     mkApp
     ;
+
 }