anchor.diff:  { supportedSystems, packageSet ? (import ./all-packages.nix) }:
  +
  +with import ../../lib;
   
   rec {
   
  
positive.code:      platform as an argument .  We return an attribute set containing
       a derivation for each supported platform, i.e. ‘{ x86_64-linux =
       f pkgs_x86_64_linux; i686-linux = f pkgs_i686_linux; ... }’. */
    testOn = systems: f: pkgs.lib.genAttrs
      (pkgs.lib.filter (x: pkgs.lib.elem x supportedSystems) systems)
      (system: f (pkgsFor system));
  
  
  
negative.code: 
  
    /* Common platform groups on which to test packages. */
    inherit (platforms) unix linux darwin cygwin allBut all mesaPlatforms;
  
    /* Platform groups for specific kinds of applications. */
    x11Supported = linux;
  