anchor.diff: -{ stdenv, writeText, erlang, rebar3, openssl,
  +{ stdenv, erlang, rebar3WithPlugins, openssl,
     lib }:
   
   { name, version
   , src
  +, beamDeps ? []
  +, buildPlugins ? []
   , checkouts ? null
   , releaseType
   , buildInputs ? []
  
positive.code:     (_: v: v != null)
      { inherit setupHook configurePhase buildPhase installPhase; };
  
    pkg = self: stdenv.mkDerivation (attrs // {
  
      name = "${name}-${version}";
      inherit version;
  
      buildInputs = buildInputs ++ [ erlang rebar3 openssl ];
      propagatedBuildInputs = [checkouts];
  
      dontStrip = true;
  
      inherit src;
  
      setupHook = writeText "setupHook.sh" ''
         addToSearchPath ERL_LIBS "$1/lib/erlang/lib/"
      '';
  
      configurePhase = ''
        runHook preConfigure
        ${if checkouts != null then
            "cp --no-preserve=all -R ${checkouts}/_checkouts ."
          else
            ""}
        runHook postConfigure
      '';
  
  
negative.code: , buildPhase ? null
  , configurePhase ? null
  , meta ? {}
  , ... }@attrs:
  
  with lib;
  