diff --git a/krebs/5pkgs/exim.nix b/krebs/5pkgs/exim.nix
deleted file mode 100644
index 4bb69267c..000000000
--- a/krebs/5pkgs/exim.nix
+++ /dev/null
@@ -1,19 +0,0 @@
-diff --git a/pkgs/servers/mail/exim/default.nix b/pkgs/servers/mail/exim/default.nix
-index 0918e30..5b7a587 100644
---- a/pkgs/servers/mail/exim/default.nix
-+++ b/pkgs/servers/mail/exim/default.nix
-@@ -1,11 +1,11 @@
- { coreutils, fetchurl, db, openssl, pcre, perl, pkgconfig, stdenv }:
- 
- stdenv.mkDerivation rec {
--  name = "exim-4.87";
-+  name = "exim-4.88";
- 
-   src = fetchurl {
--    url = "http://mirror.switch.ch/ftp/mirror/exim/exim/exim4/${name}.tar.bz2";
--    sha256 = "1jbxn13shq90kpn0s73qpjnx5xm8jrpwhcwwgqw5s6sdzw6iwsbl";
-+    url = "ftp://ftp.exim.org/pub/exim/exim4/${name}.tar.bz2";
-+    sha256 = "0bca3wb45hl7h8m8bpvsmrmqa07jhbhqyigs9pl29hhzwgbmz78i";
-   };
- 
-   buildInputs = [ coreutils db openssl pcre perl pkgconfig ];
diff --git a/lib/types.nix b/lib/types.nix
index edd48c35b..8a3c76483 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -5,7 +5,7 @@ let
     all any concatMapStringsSep concatStringsSep const filter flip genid
     hasSuffix head isInt isString length match mergeOneOption mkOption
     mkOptionType optional optionalAttrs optionals range splitString
-    stringLength tail typeOf;
+    stringLength substring typeOf;
   inherit (lib.types)
     attrsOf bool either enum int listOf nullOr path str string submodule;
 in
@@ -430,23 +430,23 @@ rec {
   };
 
   # POSIX.1‐2013, 3.2 Absolute Pathname
-  # TODO normalize slashes
-  # TODO two slashes
   absolute-pathname = mkOptionType {
     name = "POSIX absolute pathname";
-    check = x: let xs = splitString "/" x; xa = head xs; in
-         isString x
-      && stringLength x > 0
-      && (xa == "/" || (xa == "" && all filename.check (tail xs)));
+    check = x: isString x && substring 0 1 x == "/" && pathname.check x;
     merge = mergeOneOption;
   };
 
   # POSIX.1‐2013, 3.267 Pathname
-  # TODO normalize slashes
   pathname = mkOptionType {
     name = "POSIX pathname";
-    check = x: let xs = splitString "/" x; in
-      isString x && all filename.check (if head xs == "" then tail xs else xs);
+    check = x:
+      let
+        # The filter is used to normalize paths, i.e. to remove duplicated and
+        # trailing slashes.  It also removes leading slashes, thus we have to
+        # check for "/" explicitly below.
+        xs = filter (s: stringLength s > 0) (splitString "/" x);
+      in
+        isString x && (x == "/" || (length xs > 0 && all filename.check xs));
     merge = mergeOneOption;
   };