anchor.diff:        if sha256 != "" then "sha256" else if sha1 != "" then "sha1" else "md5";
     outputHash = if outputHash != "" then outputHash else
         if sha256 != "" then sha256 else if sha1 != "" then sha1 else md5;
  +  outputHashMode = if executable then "recursive" else "flat";
     
     inherit system url;
   
  
positive.code: let
  
    builder = builtins.toFile "fetchurl.sh"
      ''
        echo "downloading $url into $out"
        ${curl} --fail --location --max-redirs 20 --insecure "$url" > "$out"
      '';
  
  in
      
  
negative.code: with import <nix/config.nix>;
  
  {system ? builtins.currentSystem, url, outputHash ? "", outputHashAlgo ? "", md5 ? "", sha1 ? "", sha256 ? "", executable ? false}:
  
  assert (outputHash != "" && outputHashAlgo != "")
      || md5 != "" || sha1 != "" || sha256 != "";
  