NixOS · Jul 9, 2017
diff --git a/‎doc/cross-compilation.xml
+3-2 b/‎doc/cross-compilation.xml
+3-2
diff --git a/‎lib/lists.nix
+8 b/‎lib/lists.nix
+8
@@ -37,8 +37,9 @@
     </para>
     <para>
       In Nixpkgs, these three platforms are defined as attribute sets under the names <literal>buildPlatform</literal>, <literal>hostPlatform</literal>, and <literal>targetPlatform</literal>.
-      All three are always defined at the top level, so one can get at them just like a dependency in a function that is imported with <literal>callPackage</literal>:
-      <programlisting>{ stdenv, buildPlatform, hostPlatform, fooDep, barDep, .. }: ...</programlisting>
+      All three are always defined as attributes in the standard environment, and at the top level. That means one can get at them just like a dependency in a function that is imported with <literal>callPackage</literal>:
+      <programlisting>{ stdenv, buildPlatform, hostPlatform, fooDep, barDep, .. }: ...buildPlatform...</programlisting>, or just off <varname>stdenv</varname>:
+      <programlisting>{ stdenv, fooDep, barDep, .. }: ...stdenv.buildPlatform...</programlisting>.
     </para>
     <variablelist>
       <varlistentry>
 
@@ -477,4 +477,12 @@ rec {
   */
   subtractLists = e: filter (x: !(elem x e));
 
+  /* Test if two lists have no common element.
+     It should be slightly more efficient than (intersectLists a b == [])
+  */
+  mutuallyExclusive = a: b:
+    (builtins.length a) == 0 ||
+    (!(builtins.elem (builtins.head a) b) &&
+     mutuallyExclusive (builtins.tail a) b);
+
 }