anchor.diff:  
     buildInputs = buildInputs ++ [ erlang rebar3 openssl libyaml ];
   
  -  postPatch = ''
  +  postPatch = let
  +    registrySnapshot = fetchFromGitHub {
  +      owner = "gleber";
  +      repo = "hex-pm-registry-snapshots";
  +      rev = "48147b0";
  +      sha256 = "0ibfnhrhbka4n6wkhs99fpy3sjab54ip37jgvx2hcfhfr4pxhbxw";
  +    };
  +  in ''
       rm -f rebar rebar3
       if [ -e "src/${name}.app.src" ]; then
         sed -i -e 's/{ *vsn *,[^}]*}/{vsn, "${version}"}/' "src/${name}.app.src"
  
positive.code:     ${if compilePorts then ''
      echo "{plugins, [pc]}." >> rebar.config
      '' else ''''}
  
      ${postPatch}
    '';
  
negative.code:     };
    in ''
      runHook preConfigure
      ${concatMapStrings (dep: ''
        header "linking erlang dependency ${dep}"
        ln -s "${dep}" "_build/default/lib/${dep.packageName}"
  