anchor.diff:  
   stdenv.mkDerivation  rec {
     pname = "mpich";
  -  version = "3.4.3";
  +  version = "4.0";
   
     src = fetchurl {
       url = "https://www.mpich.org/static/downloads/${version}/mpich-${version}.tar.gz";
  -    sha256 = "1msg5i2mcmjix5pvpa84dwmlqpqm3206frl1060k342i62gxhm41";
  +    sha256 = "0r7zzcj8b9dbf5lp2d81wcvffi38c1zchkgzyxckk51adv4ijx6z";
     };
   
     configureFlags = [
  
positive.code: { stdenv, lib, fetchurl, perl, gfortran
  , openssh, hwloc
  # either libfabric or ucx work for ch4backend on linux. On darwin, neither of
  # these libraries currently build so this argument is ignored on Darwin.
  , ch4backend
  
negative.code: 
    enableParallelBuilding = true;
  
    nativeBuildInputs = [ gfortran python3 ];
    buildInputs = [ perl openssh hwloc ]
      ++ lib.optional (!stdenv.isDarwin) ch4backend;
  
  