anchor.diff:  assert enableGUI -> libGLU != null && libGL != null && xorg != null && fltk != null;
   
   stdenv.mkDerivation rec {
  -  name = "${attr}-${version}";
  -  attr = if enableGUI then "giac-with-xcas" else "giac";
  +  pname = "giac${lib.optionalString enableGUI "-with-xcas"}";
     version = "1.5.0-21"; # TODO try to remove preCheck phase on upgrade
   
     src = fetchurl {
  
positive.code:     description = "A free computer algebra system (CAS)";
      homepage = "https://www-fourier.ujf-grenoble.fr/~parisse/giac.html";
      license = licenses.gpl3Plus;
      platforms = platforms.unix;
      maintainers = [ maintainers.symphorien ];
    };
  }
  
negative.code: { stdenv, lib, fetchurl, fetchpatch, texlive, bison, flex, liblapack
  , gmp, mpfr, pari, ntl, gsl, blas, mpfi, ecm, glpk, nauty
  , readline, gettext, libpng, libao, gfortran, perl
  , enableGUI ? false, libGL ? null, libGLU ? null, xorg ? null, fltk ? null
  