anchor.diff:    src = fetch "libcxxabi" "1zcqxsdjhawgz1cvpk07y3jl6fg9p3ay4nl69zsirqb2ghgyhhb2";
   
     nativeBuildInputs = [ cmake ];
  +  buildInputs = lib.optional (!stdenv.isDarwin && !stdenv.isFreeBSD && !stdenv.hostPlatform.isWasm) libunwind;
   
     postUnpack = ''
       unpackFile ${libcxx.src}
  
positive.code:     patch -p1 -d $(ls -d libcxx-*) -i ${../../libcxx-0001-musl-hacks.patch}
    '';
  
    cmakeFlags =
       lib.optional standalone "-DLLVM_ENABLE_LIBCXX=ON" ++
       lib.optional (!enableShared) "-DLIBCXXABI_ENABLE_SHARED=OFF";
  
    installPhase = if stdenv.isDarwin
      then ''
  
negative.code: { lib, stdenv, cmake, fetch, libcxx, libunwind, llvm, version
  , standalone ? stdenv.hostPlatform.useLLVM or false
    # on musl the shared objects don't build
  , enableShared ? !stdenv.hostPlatform.isStatic
  }:
  