anchor.diff:  
     enableParallelBuilding = true;
   
  -  nativeBuildInputs = [ gfortran ];
  +  nativeBuildInputs = [ gfortran python3 ];
     buildInputs = [ perl openssh hwloc ]
       ++ lib.optional (!stdenv.isDarwin) 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: { stdenv, lib, fetchurl, perl, gfortran
  , 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
  