anchor.diff:          </programlisting>
           See <code>nixpkgs.localSystem</code> for more information.
   
  +        Ignored when <code>nixpkgs.localSystem</code> is set.
           Ignored when <code>nixpkgs.pkgs</code> is set.
         '';
       };
  
positive.code: { config, lib, pkgs, ... }:
  
  with lib;
  
  let
    cfg = config.nixpkgs;
  
    isConfig = x:
      builtins.isAttrs x || lib.isFunction x;
  
negative.code: 
      system = mkOption {
        type = types.str;
        default = cfg.localSystem.system
          or opts.localSystem.default.system;
        example = "i686-linux";
        description = ''
          Specifies the Nix platform type on which NixOS should be built.
  