anchor.diff:              nix-env -iA nixos.pkgs.git || nix-env -i git
           fi
   
  -        # Move any old nixos or nixpkgs directories out of the way.
  +        # Move any old nixpkgs directories out of the way.
           backupTimestamp=$(date "+%Y%m%d%H%M%S")
  -
  -        if [ -e nixos -a ! -e nixos/.git ]; then
  -            mv nixos nixos-$backupTimestamp
  -        fi
   
           if [ -e nixpkgs -a ! -e nixpkgs/.git ]; then
               mv nixpkgs nixpkgs-$backupTimestamp
           fi
   
           # Check out the NixOS and Nixpkgs sources.
  -        git clone git://github.com/NixOS/nixos.git nixos
           git clone git://github.com/NixOS/nixpkgs.git nixpkgs
         '';
      };
  
positive.code: # This module generates the nixos-checkout script, which replaces the
  # NixOS and Nixpkgs source trees in /etc/nixos/{nixos,nixpkgs} with
  # Git checkouts.
  
  {config, pkgs, ...}:
  
  with pkgs.lib;
  
  
negative.code:         if [ "$1" == "-h" ] || [ "$1" == "--help" ]; then
            echo "Usage: `basename $0` [PREFIX]. See NixOS Manual for more info."
            exit 0
          fi
  
          prefix="$1"
          if [ -z "$prefix" ]; then prefix=/etc/nixos; fi
  