diff --git a/28.nix b/28.nix
new file mode 100644
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--- /dev/null
+++ b/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))
\ No newline at end of file