diff --git a/25.nix b/25.nix
new file mode 100644
index 0000000..8342686
--- /dev/null
+++ b/25.nix
@@ -0,0 +1,11 @@
+with import <nixpkgs> {};
+let
+  func = x: y: assert (x==2) || abort "x has to be 2 or it won't work!"; x + y;
+  n = -1; # only modify this line
+in
+
+assert (lib.isInt n) || abort "Type error since supplied argument is no int!";
+
+rec {
+  ex00 = func (n+3) 3;
+}
\ No newline at end of file