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