anchor.diff:          []);
       in if res.success then res.value else []
       );
  -
  -
  -  /* Common platform groups on which to test packages. */
  -  inherit (platforms) unix linux allBut all;
  -
   }
  
positive.code:   testOn = systems: f: genAttrs
      (filter (x: elem x supportedSystems) systems) (system: hydraJob' (f (pkgsFor system)));
  
  
    /* Similar to the testOn function, but with an additional
       'crossSystem' parameter for allPackages, defining the target
       platform for cross builds. */
    testOnCross = crossSystem: systems: f: {system ? builtins.currentSystem}:
      if elem system systems
      then f (allPackages { inherit system crossSystem; })
      else {};
  
  
    /* Given a nested set where the leaf nodes are lists of platforms,
       map each leaf node to `testOn [platforms...] (pkgs:
       pkgs.<attrPath>)'. */
    mapTestOn = mapAttrsRecursive
      (path: systems: testOn systems (pkgs: getAttrFromPath path pkgs));
  
  
    /* Similar to the testOn function, but with an additional 'crossSystem'
     * parameter for allPackages, defining the target platform for cross builds,
     * and triggering the build of the host derivation (cross built - crossDrv). */
    mapTestOnCross = crossSystem: mapAttrsRecursive
      (path: systems: testOnCross crossSystem systems
        (pkgs: addMetaAttrs { maintainers = crossMaintainers; } (getAttrFromPath path pkgs)));
  
  
    /* Recursively map a (nested) set of derivations to an isomorphic
       set of meta.platforms values. */
  
negative.code: { supportedSystems
  , allPackages
  , allowTexliveBuilds ? false
  , scrubJobs ? true
  }:
  
  with import ../../lib;
  
  let
    allPackages' = args: allPackages args // {
      config = {
        allowUnfree = false;
        inHydra = true;
        inherit allowTexliveBuilds;
      };
      stdenv = null;
    };
  
    pkgs_x86_64-linux = allPackages' {
      targetSystem = "x86_64-linux";
      hostSystem = "x86_64-linux";
    };
    pkgs_i686-linux = allPackages' {
      targetSystem = "i686-linux";
      hostSystem = "x86_64-linux";
    };
  in
  rec {
    /* !!! Hack: poor man's memoisation function.  Necessary to prevent
       Nixpkgs from being evaluated again and again for every
       job/platform pair. */
    pkgsFor = { targetSystem, hostSystem }:
      if targetSystem == "x86_64-linux" && hostSystem == "x86_64-linux" then
        pkgs_x86_64-linux
      else if targetSystem == "i686-linux" && hostSystem == "i686-linux" then
        pkgs_i686-linux
      else
        abort "unsupported system type: ${targetSystem} built by ${hostSystem}";
  
    hydraJob' = if scrubJobs then hydraJob else id;
  
    /* Build a package on the given set of platforms.  The function `f'
       is called for each supported platform with Nixpkgs for that
  