anchor.diff:      };
   
     # Trivial Nix expression to build remotely.
  -  expr = config: pkgs.writeText "expr.nix"
  +  expr = config: nr: pkgs.writeText "expr.nix"
       ''
         let utils = builtins.storePath ${config.system.build.extraUtils}; in
         derivation {
  -        name = "hello";
  +        name = "hello-${toString nr}";
           system = "i686-linux";
           PATH = "''${utils}/bin";
           builder = "''${utils}/bin/sh";
  -        args = [ "-c" "echo Hello; mkdir $out; cat /proc/sys/kernel/hostname > $out/host" ];
  +        args = [ "-c" "echo Hello; mkdir $out; cat /proc/sys/kernel/hostname > $out/host; sleep 3" ];
         }
       '';
   
  
positive.code:       $client->succeed("chmod 600 /root/.ssh/id_dsa");
  
        # Install the SSH key on the slaves.
        $slave1->succeed("mkdir -m 700 /root/.ssh");
        $slave1->copyFileFromHost("key.pub", "/root/.ssh/authorized_keys");
        $slave1->waitForJob("sshd");
        $client->succeed("ssh -o StrictHostKeyChecking=no slave1 'echo hello world'");
  
        # Perform a build and check that it was performed on the slave.
        my $out = $client->succeed("nix-build ${expr nodes.client.config}");
        $slave1->succeed("test -e $out");
      '';
  
  }
  
negative.code: 
    nodes =
      { slave1 = slave;
        slave2 = slave;
  
        client =
          { config, pkgs, ... }:
  