anchor.diff:  { stdenv
   , elfutils
   , fetchzip
  +, perl
   
   , kernel
   , libmnl
  
positive.code:   src = fetchzip {
      version = 6;
      url = "https://git.zx2c4.com/WireGuard/snapshot/WireGuard-${rev}.tar.xz";
      multihash = "QmYLWtAL71T5K1Y8VuF6HaJJwG77Lo5PoXHM6J6G3gxbAG";
      sha256 = "e104063c86d669871e8f6295c8db7429c98f1f39f585f17f22c799cc35713a4a";
    };
  
    nativeBuildInputs = optionals (kernel != null) [
      elfutils
    ];
  
    buildInputs = optionals (kernel == null) [
  
negative.code:     optionals
      optionalString;
  
    rev = "79b5151f24ef5c36ede11968e4a00b9d68468fba";
    date = "2018-12-18";
  in
  stdenv.mkDerivation {
    name = "wireguard-${date}${optionalString (kernel != null) "-${kernel.version}"}";
  