Add '28.nix'

28.nix

@@ -0,0 +1,11 @@
+with import <nixpkgs> { };
+let
+  list = ["a" "b" "a" "c" "d" "a"];
+  countA = lib.fold (x: y: if x != "a" then y else y + 1) 0;
+in
+rec {
+  example = lib.fold (x: y: x + y) "" ["a" "b" "c"]; #is "abc"
+  result = countA list; #should be 3
+}
+
+# fold f z [x_1 x_2 ... x_n] == f x_1 (f x_2 ... (f x_n z))