anchor.diff:  { stdenv, lib, fetchurl, fetchFromSavannah, fetchFromGitHub
   , zlib, openssl, gdbm, ncurses, readline, groff, libyaml, libffi, autoreconfHook, bison
  -, autoconf, darwin ? null
  +, autoconf
   } @ args:
   
   let
  
positive.code:       , libyaml, yamlSupport ? true
        , libffi, fiddleSupport ? true
        , autoreconfHook, bison, autoconf
        , darwin ? null
        }:
        stdenv.mkDerivation rec {
          inherit version;
  
negative.code:         '';
  
          configureFlags = ["--enable-shared" "--enable-pthread"]
            ++ op useRailsExpress "--with-baseruby=${baseruby}/bin/ruby";
  
          installFlags = stdenv.lib.optionalString docSupport "install-doc";
          # Bundler tries to create this directory
  