anchor.diff:      optionals
       platforms;
   
  -  version = "4.19";
  +  version = "4.20";
   
     baseUrls = [
       "https://ftp.mozilla.org/pub/mozilla.org/nspr/releases/v${version}/src"
  
positive.code:   src = fetchurl {
      urls = map (n: "${n}/nspr-${version}.tar.gz") baseUrls;
      hashOutput = false;
      sha256 = "2ed95917fa2277910d1d1cf36030607dccc0ba522bba08e2af13c113dcd8f729";
    };
  
    prePatch = ''
  
negative.code: 
    passthru = {
      srcVerification = fetchurl {
        failEarly = true;
        inherit (src)
          outputHash
          outputHashAlgo
          urls;
        fullOpts = {
          sha256Urls = map (n: "${n}/SHA256SUMS") baseUrls;
        };
      };
    };
  
  