anchor.diff:    urls = tail urls_;
   
     system = "builtin";
  +})) // {
  +  inherit
  +    multihash
  +    urls;
   }
  
positive.code: let
    inherit (lib)
      concatStringsSep
      hasPrefix
      head
      optionals
  
negative.code:       ) urls) ++ optionals (multihash != "")
      (map (n: "${n}/ipfs/${multihash}") mirrors.ipfs-nocache);
  in
  (filterAttrs (n: _: n != "url") (derivation {
    inherit
      name
      outputHash
  