anchor.diff:    doCheck = true;
     checkPhase = ''
       runHook preCheck
  +    # Patch shebangs for playTests
  +    patchShebangs ../programs/zstdgrep
       ctest -R playTests # The only relatively fast test.
       runHook postCheck
     '';
  
positive.code: { stdenv, fetchFromGitHub, fetchpatch, cmake, gnugrep
  , fixDarwinDylibNames
  , file
  , legacySupport ? false
  
negative.code: 
    nativeBuildInputs = [ cmake ]
     ++ stdenv.lib.optional stdenv.isDarwin fixDarwinDylibNames;
    buildInputs = [ bash ];
  
    patches = [
      ./playtests-darwin.patch
  