diff --git a/horizon-spec/example.dhall b/horizon-spec/example.dhall
index 0e43c88fecfcf2ccc57c4905afb859a8786b3b0c..0fc56ff4a3a7d54a7fee58c51d480982dd84b0b6 100644
--- a/horizon-spec/example.dhall
+++ b/horizon-spec/example.dhall
@@ -1,6 +1,6 @@
 let H =
       ./package.dhall
-        sha256:cb66c78876ebce1b47a192b7ae9c5eb58e77d12f243cf74e0e3a87022adf5faa
+        sha256:f52fc6fc1f8569c1afbc6be778f751242592127164666ffe2e8a9cbe7f2059a8
 
 let packages =
       { Cabal = H.callHackage "Cabal" "3.8.1.0"
@@ -144,7 +144,8 @@ let packages =
       , xml = H.callHackage "xml" "1.3.14"
       }
 
-in  { packageSetFile = "initial-packages.nix"
-    , packagesDir = "pkgs/"
-    , packageSet = { compiler = "ghc-9.4.4", packages = toMap packages }
-    }
+in  H.HorizonExport.MakePackageSet
+      { packageSetFile = "initial-packages.nix"
+      , packagesDir = "pkgs/"
+      , packageSet = { compiler = "ghc-9.4.4", packages = toMap packages }
+      }
diff --git a/horizon-spec/package.dhall b/horizon-spec/package.dhall
index e85993a6f95966f215cfea1f47d42e4bbcf95c91..3f0c134a5126fdcfae77550d286c3965f564f379 100644
--- a/horizon-spec/package.dhall
+++ b/horizon-spec/package.dhall
@@ -76,12 +76,22 @@ let PackageList = List PackageEntry
 
 let PackageSet = { compiler : Compiler, packages : PackageList }
 
-let HorizonExport =
+let Overlay = PackageSet
+
+let PackageSetExportSettings =
       { packagesDir : Directory
       , packageSetFile : FilePath
       , packageSet : PackageSet
       }
 
+let OverlayExportSettings =
+      { packagesDir : Directory, overlayFile : FilePath, overlay : Overlay }
+
+let HorizonExport =
+      < MakePackageSet : PackageSetExportSettings
+      | MakeOverlay : OverlayExportSettings
+      >
+
 let callHackage
     : Name → Version → HaskellPackage.Type
     = λ(name : Name) →
@@ -129,8 +139,11 @@ in  { Attr
     , HorizonExport
     , Modifiers
     , Name
+    , OverlayExportSettings
+    , Overlay
     , PackageEntry
     , PackageList
+    , PackageSetExportSettings
     , PackageSet
     , Policy
     , Revision