anchor.diff: -{ stdenv, fetchurl, attr, perl, pam ? null }:
  +{ stdenv, buildPackages, fetchurl, attr, perl, pam ? null }:
   assert pam != null -> stdenv.isLinux;
   
   stdenv.mkDerivation rec {
  
positive.code:   makeFlags = [
      "lib=lib"
      (stdenv.lib.optional (pam != null) "PAM_CAP=yes")
    ];
  
    prePatch = ''
  
negative.code:   outputs = [ "out" "dev" "lib" "man" "doc" ]
      ++ stdenv.lib.optional (pam != null) "pam";
  
    nativeBuildInputs = [ perl buildPackages.stdenv.cc ];
  
    buildInputs = [ pam ];
  
  