twelf: Fix Emacs mode syntax for Emacs ≥ 27

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

pkgs/by-name/tw/twelf/package.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchurl, pkg-config, smlnj, rsync }:
+{ lib, stdenv, fetchurl, fetchpatch, pkg-config, smlnj, rsync }:
 
 stdenv.mkDerivation rec {
   pname = "twelf";
@@ -9,6 +9,14 @@ stdenv.mkDerivation rec {
     sha256 = "0fi1kbs9hrdrm1x4k13angpjasxlyd1gc3ys8ah54i75qbcd9c4i";
   };
 
+  patches = [
+    # Fix Emacs old-style backquotes: https://github.com/standardml/twelf/pull/3
+    (fetchpatch {
+      url = "https://github.com/standardml/twelf/commit/7b3f3dbb8b8ec8d16d843875fce1e2bd6a50e3ae.patch";
+      hash = "sha256-cSrgQFRPL+4zRtFXv3rLsAasjLfal0TZpXasEUtUSHc=";
+    })
+  ];
+
   nativeBuildInputs = [ pkg-config ];
   buildInputs = [ smlnj rsync ];