anchor.diff:          '';
   
           configureFlags = ["--enable-shared" "--enable-pthread"]
  -          ++ op useRailsExpress "--with-baseruby=${baseruby}/bin/ruby"
  -          ++ ops stdenv.isDarwin [
  -            # on darwin, we have /usr/include/tk.h -- so the configure script detects
  -            # that tk is installed
  -            "--with-out-ext=tk"
  -            # on yosemite, "generating encdb.h" will hang for a very long time without this flag
  -            "--with-setjmp-type=setjmp"
  -          ];
  +          ++ op useRailsExpress "--with-baseruby=${baseruby}/bin/ruby";
   
           installFlags = stdenv.lib.optionalString docSupport "install-doc";
           # Bundler tries to create this directory
  
positive.code: { stdenv, lib, fetchurl, fetchFromSavannah, fetchFromGitHub
  , zlib, openssl, gdbm, ncurses, readline, groff, libyaml, libffi, autoreconfHook, bison
  , autoconf, darwin ? null
  } @ args:
  
  let
  
negative.code:       , libyaml, yamlSupport ? true
        , libffi, fiddleSupport ? true
        , autoreconfHook, bison, autoconf
        }:
        stdenv.mkDerivation rec {
          inherit version;
  