[Nix-dev] how to overwrite a derivation in nix

[Sergey]

Hi, list! Does nixpkgs library contain a function able to overwrite an 
existing derivation? I would like the following usage pattern to be 
possible:

with pkgs; {

thunar_with_whistles = overwrite pkgs.thunar (old:{
   patches = old.patches ++ [ ./add_whistles.patch ];
});

}

I may want to add a new patch (like in the pattern above), add new build 
phase or change some meta field.


Regards,
Sergey