diff --git a/.travis.yml b/.travis.yml
index e1d6117..8065717 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -22,13 +22,13 @@ install:
  - travis_retry nix-channel --update
 
 script:
- - outs=$(nix-build buildable.nix) && echo Produced $outs
+ - nix-build ci.nix -A buildOutputs
  - nix eval -f default.nix 'lib'
  - nix eval -f default.nix 'modules'
  - nix eval -f default.nix 'overlays'
 
 after_success:
-  - if [ -n "${CACHIX_CACHE}" ]; then cachix push "${CACHIX_CACHE}" $outs; fi
+  - if [ -n "${CACHIX_CACHE}" ]; then nix-build ci.nix -A cacheOutputs | cachix push "${CACHIX_CACHE}"; fi
   - if [ "false" = "${TRAVIS_PULL_REQUEST}" -a "master" = "${TRAVIS_BRANCH}" ]; then
       curl -XPOST "https://nur-update.herokuapp.com/update?repo=${NUR_REPO}"; fi
 
diff --git a/buildable.nix b/buildable.nix
deleted file mode 100644
index 2b61b1f..0000000
--- a/buildable.nix
+++ /dev/null
@@ -1,28 +0,0 @@
-# This file filters out all the unbuildable packages from your package set.
-# It's what gets built by CI, so if you correctly mark broken/unfree packages
-# as such your CI will not try to build them and the buildable packages will
-# be added to the cache.
-{ pkgs ? import <nixpkgs> {} }:
-
-let
-  filterSet =
-    (f: g: s: builtins.listToAttrs
-      (map
-        (n: { name = n; value = builtins.getAttr n s; })
-        (builtins.filter
-          (n: f n && g (builtins.getAttr n s))
-          (builtins.attrNames s)
-        )
-      )
-    );
-  isReserved = n: builtins.elem n ["lib" "overlays" "modules"];
-  isBroken = p: p.meta.broken or false;
-  isFree = p: p.meta.license.free or true;
-in filterSet
-     (n: !(isReserved n)) # filter out non-packages
-     (p: (builtins.isAttrs p)
-       && !(isBroken p)
-       && isFree p
-     )
-     (import ./default.nix { inherit pkgs; })
-
diff --git a/ci.nix b/ci.nix
new file mode 100644
index 0000000..1377d43
--- /dev/null
+++ b/ci.nix
@@ -0,0 +1,56 @@
+# This file provides all the buildable and cacheable packages and
+# package outputs in you package set. These are what gets built by CI,
+# so if you correctly mark packages as
+#
+# - broken (using `meta.broken`),
+# - unfree (using `meta.license.free`), and
+# - locally built (using `preferLocalBuild`)
+#
+# then your CI will be able to build and cache only those packages for
+# which this is possible.
+
+{ pkgs ? import <nixpkgs> {} }:
+
+with builtins;
+
+let
+
+  isReserved = n: n == "lib" || n == "overlays" || n == "modules";
+  isDerivation = p: isAttrs p && p ? type && p.type == "derivation";
+  isBuildable = p: !(p.meta.broken or false) && p.meta.license.free or false;
+  isCacheable = p: !(p.preferLocalBuild or false);
+  shouldRecurseForDerivations = p: isAttrs p && p.recurseForDerivations or false;
+
+  nameValuePair = n: v: { name = n; value = v; };
+
+  concatMap = builtins.concatMap or (f: xs: concatLists (map f xs));
+
+  flattenPkgs = s:
+    let
+      f = p:
+        if shouldRecurseForDerivations p then flattenPkgs p
+        else if isDerivation p then [p]
+        else [];
+    in
+      concatMap f (attrValues s);
+
+  outputsOf = p: map (o: p.${o}) p.outputs;
+
+  nurAttrs = import ./default.nix { inherit pkgs; };
+
+  nurPkgs =
+    flattenPkgs
+    (listToAttrs
+    (map (n: nameValuePair n nurAttrs.${n})
+    (filter (n: !isReserved n)
+    (attrNames nurAttrs))));
+
+in
+
+rec {
+  buildPkgs = filter isBuildable nurPkgs;
+  cachePkgs = filter isCacheable buildPkgs;
+
+  buildOutputs = concatMap outputsOf buildPkgs;
+  cacheOutputs = concatMap outputsOf cachePkgs;
+}
diff --git a/overlay.nix b/overlay.nix
index bda4332..122729d 100644
--- a/overlay.nix
+++ b/overlay.nix
@@ -1,23 +1,18 @@
-# You can use this file as a nixpkgs overlay.
-# It's useful in the case where you don't want to add the whole NUR namespace
-# to your configuration.
+# You can use this file as a nixpkgs overlay. This is useful in the
+# case where you don't want to add the whole NUR namespace to your
+# configuration.
 
 self: super:
 
 let
-  filterSet =
-    (f: g: s: builtins.listToAttrs
-      (map
-        (n: { name = n; value = builtins.getAttr n s; })
-        (builtins.filter
-          (n: f n && g (builtins.getAttr n s))
-          (builtins.attrNames s)
-        )
-      )
-    );
-  isReserved = n: builtins.elem n ["lib" "overlays" "modules"];
-in filterSet
-     (n: !(isReserved n)) # filter out non-packages
-     (p: true) # all packages are ok
-     (import ./default.nix { pkgs = super; })
 
+  isReserved = n: n == "lib" || n == "overlays" || n == "modules";
+  nameValuePair = n: v: { name = n; value = v; };
+  nurAttrs = import ./default.nix { pkgs = super; };
+
+in
+
+  builtins.listToAttrs
+  (map (n: nameValuePair n nurAttrs.${n})
+  (builtins.filter (n: !isReserved n)
+  (builtins.attrNames nurAttrs)))