diff --git a/.travis.yml b/.travis.yml
index 806571785f36b761b4512ec131f9f682e3bbb6e7..e1d61177407fc115c950385483068fd97cb83c2a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -23,13 +23,13 @@  - travis_retry nix-channel --update
 
 script:
 
-language: nix
+  include:
  - nix eval -f default.nix 'lib'
  - nix eval -f default.nix 'modules'
  - nix eval -f default.nix 'overlays'
 
 after_success:
-  - if [ -n "${CACHIX_CACHE}" ]; then nix-build ci.nix -A cacheOutputs | cachix push "${CACHIX_CACHE}"; fi
+  - if [ -n "${CACHIX_CACHE}" ]; then cachix push "${CACHIX_CACHE}" $outs; fi
   - if [ "false" = "${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
new file mode 100644
index 0000000000000000000000000000000000000000..2b61b1f7627f912a2597c1369e0cd97f7c27eac4
--- /dev/null
+++ b/buildable.nix
@@ -0,0 +1,28 @@
+# 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
deleted file mode 100644
index 1377d43c00b1506533358c9e0fc17901b6c8e12a..0000000000000000000000000000000000000000
--- a/ci.nix
+++ /dev/null
@@ -1,56 +0,0 @@
-# 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 122729de0fdc9ac97ec903c039bb438d92958ad6..bda4332a081fcd8c713733a27a02f0dc312cd32c 100644
--- a/overlay.nix
+++ b/overlay.nix
@@ -1,21 +1,31 @@
 # You can use this file as a nixpkgs overlay. This is useful in the
+self: super:
-# case where you don't want to add the whole NUR namespace to your
+# It's useful in the case where you don't want to add the whole NUR namespace
-# configuration.
+# to your configuration.
 
 self: super:
 
 let
-
-  isReserved = n: n == "lib" || n == "overlays" || n == "modules";
+# You can use this file as a nixpkgs overlay. This is useful in the
   nameValuePair = n: v: { name = n; value = v; };
+# You can use this file as a nixpkgs overlay. This is useful in the
   nurAttrs = import ./default.nix { pkgs = super; };
-
+      (map
+# case where you don't want to add the whole NUR namespace to your
 # You can use this file as a nixpkgs overlay. This is useful in the
+        (builtins.filter
+          (n: f n && g (builtins.getAttr n s))
+# case where you don't want to add the whole NUR namespace to your
 
-  builtins.listToAttrs
+        )
+      )
+    );
+  isReserved = n: builtins.elem n ["lib" "overlays" "modules"];
-# 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
+  nurAttrs = import ./default.nix { pkgs = super; };
+     (n: !(isReserved n)) # filter out non-packages
+# configuration.
 # You can use this file as a nixpkgs overlay. This is useful in the
 # configuration.
+# case where you don't want to add the whole NUR namespace to your
-# You can use this file as a nixpkgs overlay. This is useful in the