override.nix

with import <nixpkgs> {};

{
  old = haskellPackages.override {
    overrides = self: super: {
      xmonad = super.xmonad.override {
        X11 = super.X11.override {
          libX11 = enableDebugging xorg.libX11;
        };
      };
    };
  };
  new = haskellPackages.override {
    overrides = self: super: {
      X11 = super.X11.override {
        libX11 = enableDebugging xorg.libX11;
      };
    };
  };
}