anchor.diff:  { stdenv, lib, fetchurl, perl, gfortran
  -, openssh, hwloc
  +, openssh, hwloc, python3
   # 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
  
positive.code: 
  stdenv.mkDerivation  rec {
    pname = "mpich";
    version = "3.4.3";
  
    src = fetchurl {
      url = "https://www.mpich.org/static/downloads/${version}/mpich-${version}.tar.gz";
      sha256 = "1msg5i2mcmjix5pvpa84dwmlqpqm3206frl1060k342i62gxhm41";
    };
  
    configureFlags = [
  
negative.code: 
    enableParallelBuilding = true;
  
    nativeBuildInputs = [ gfortran python3 ];
    buildInputs = [ perl openssh hwloc ]
      ++ lib.optional (!stdenv.isDarwin) ch4backend;
  
  