diff --git a/pkgs/lib/customisation.nix b/pkgs/lib/customisation.nix
index e6a26958ae2..334c8036627 100644
--- a/pkgs/lib/customisation.nix
+++ b/pkgs/lib/customisation.nix
@@ -1,4 +1,7 @@
-let lib = import ./default.nix; in
+let lib = import ./default.nix;
+    inherit (builtins) getAttr attrNames isFunction;
+
+in
 
 rec {
 
@@ -31,7 +34,10 @@ rec {
   overrideDerivation = drv: f:
     let
       # Filter out special attributes.
-      attrs = removeAttrs drv ["meta" "passthru" "outPath" "drvPath" "hostDrv" "buildDrv" "type" "override" "deepOverride" "origArgs"];
+      drop = ["meta" "passthru" "outPath" "drvPath" "hostDrv" "buildDrv" "type" "override" "deepOverride" "origArgs"]
+              # also drop functions such as .merge .override etc
+             ++ lib.filter (n: isFunction (getAttr n drv)) (attrNames drv);
+      attrs = removeAttrs drv drop;
       newDrv = derivation (attrs // (f drv));
     in newDrv //
       { meta = if drv ? meta then drv.meta else {};
diff --git a/pkgs/misc/misc.nix b/pkgs/misc/misc.nix
index fc8ab43d52f..3e4e94fbc78 100644
--- a/pkgs/misc/misc.nix
+++ b/pkgs/misc/misc.nix
@@ -1,6 +1,6 @@
 { pkgs, stdenv } :
 
-let inherit (pkgs) stdenv runCommand perl;
+let inherit (pkgs) stdenv runCommand perl lib;
 
 in
 
@@ -106,4 +106,30 @@ in
       echo "''${PATH}" > $target/PATH
     '';
   };
+
+
+  # build a debug version of a package
+  debugVersion = pkg: lib.overrideDerivation pkg (attrs: {
+
+    prePhases = ["preHook"] ++ lib.optionals (pkgs ? prePhases) pkgs.prePhases;
+
+    dontStrip = true;
+
+    NIX_STRIP_DEBUG=0;
+    CFLAGS="-ggdb -O0";
+    CXXFLAGS="-ggdb -O0";
+
+    preHook = ''
+      s=$out/src
+      mkdir -p $s; cd $s;
+      export TMP=$s
+      export TEMP=$s
+
+      for var in CFLAGS CXXFLAGS NIX_CFLAGS_COMPILE; do
+        declare -x "$var=''${!var} -ggdb -O0"
+      done
+      echo "file should tell that executable has not been strippee"
+    '';
+
+  });
 }