anchor.diff:                  build-users-group = nixbld
                   build-max-jobs = ${toString (config.nix.maxJobs)}
                   build-use-chroot = ${if config.nix.useChroot then "true" else "false"}
  -                build-chroot-dirs = /dev /dev/pts /proc /bin $(echo $extraPaths) ${builtins.toString config.nix.extraChrootPaths}
  +                build-chroot-dirs = ${toString config.nix.chrootDirs} $(echo $extraPaths)
                   $extraOptions
                   END
                 '';
  
positive.code:   ###### implementation
  
    config = {
  
      environment.etc =
        [ { # Nix configuration.
  
negative.code:         ";
        };
  
        chrootDirs = mkOption {
          default = [];
  	example = [ "/dev" "/proc" ];
  	description =
  	  ''
  	    Directories from the host filesystem to be included
  	    in the chroot.
  	  '';
        };
  
        extraOptions = mkOption {
          default = "";
          example = "
  