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{
  packageOverrides = pkgs: {
    unison = (pkgs.unison.override {
      ocaml = pkgs.ocaml_4_02;
      enableX11 = false;
    }).overrideDerivation (oldAttrs: {
      src = pkgs.fetchurl {
        url = "http://www.seas.upenn.edu/~bcpierce/unison/download/releases/unison-2.49.543/unison-2.49.543.tar.gz";
        sha512 = "3kg1vic15vi5qhczjxd8h69ibs188hcfb882hcmprwqz22dw48b4frwjwvdiwa02byxjqc26zsalz490m3yrmc6rzalgpgx1bwbrvzw";
      };
    });
  };
}